defective medical device used at Johns Hopkins Hospital in Baltimore by doctors who say they did not know that it had been recalled may have exposed more than 400 patients to bacteria that can cause a dangerous lung infection.

Two patients who were critically ill when the device was used on them have died of pneumonia. The hospital said it did not know whether the device, a bronchoscope, was at fault.

Hopkins officials said the recall procedures used by the manufacturer of the device were inadequate and had left other hospitals unaware of the danger to patients. The officials added that the procedures raised serious questions about the effectiveness of recalls of medical equipment.

The manufacturer, Olympus America in Melville, N.Y., part of the Olympus Optical Company of Tokyo, issued a statement saying that it had "initiated an immediate and vigorous investigation resulting in a prompt recall" and that notices had been sent to 2,361 health-care institutions. Olympus said it had confirmations that more than 90 percent of the recall notices had been received.

But Olympus said that since the recall was issued on Nov. 30, fewer than 40 percent of the recalled bronchoscopes had been sent back to the company for inspection and repair. The company said it had sent second notices to institutions that did not answer the first.

The Food and Drug Administration is investigating whether the recall was properly carried out, a spokesman for the agency said.

Experts involved with the case said that it showed that recall procedures for medical devices were inadequate and that the F.D.A. needed to play a stronger role in recalls.

Officials at Hopkins said the hospital had notified patients who had the diagnostic test, a broncheolar lavage, from last June 1 to Feb. 4 and had offered them free evaluations and tests for the infection.

It is not known how widespread the problem is or whether patients who had the test at other hospitals had been informed of the problem.

A bronchoscope is a camera- equipped tube passed through the nose or mouth and into the lungs to let doctors see inside the lungs and take biopsies and samples of secretions. It is often used in very sick people to diagnose lung infections and cancer. Nearly half a million people undergo the procedure each year in the United States.

Olympus, whose bronchoscopes are widely used, recalled 15 models on Nov. 30 because a loose valvelike component called a port was trapping bacteria in a spot that the usual disinfecting process could not reach. Bronchoscopes are not steam-sterilized like other instruments, but are cleaned with chemicals and washed in a special machine similar to a dishwasher.

The recall started because in September an infection-control nurse at a hospital in Tennessee noticed that an unusually high number of cultures performed on lung secretions collected from bronchoscopes were coming back positive for a bacterium called pseudomonas. Health officials have not identified the hospital. Suspecting contamination, the nurse alerted the state health department.

Epidemiologists from the department and from Vanderbilt University traced the problem to the Olympus bronchoscopes, which they said were picking up bacteria from one patient, shielding them from the disinfectants and transferring them to the next patient exposed to the scope.

The epidemiologists told Olympus, which sent recall letters to hundreds of hospitals that had bought the bronchoscopes. The company sent a copy of the letter to the F.D.A.

At a news conference yesterday, Hopkins officials said that they did not receive the Nov. 30 recall letter until late January, because instead of being sent to the department that used the bronchoscopes, it was addressed to a loading dock at another department across the street. The letter had a return receipt requested. No one had signed for it, and as far as they knew, Hopkins officials said, Olympus had not followed up.

"We would have liked to have seen a more vigorous notification process," said Dr. Paul Scheel, vice chairman of the Hopkins department of medicine.

In the meantime, doctors at Hopkins, like those in Tennessee, had begun to notice an unusually high rate of pseudomonas infections among patients exposed to bronchoscopes. Hopkins began its own investigation and developed a culturing technique based on running fluid backward through the scope to detect the bacteria.

Dr. Scheel said doctors at Hopkins began calling other hospitals to warn them of the problem, to tell them how to culture the bacteria and to alert them to the recall. He said mention of the recall was usually greeted by a long silence and then the question, "What recall?"

Dr. Scheel added, "We don't know how many of the recalled scopes are being used on a daily basis."

He said that with drug problems, the F.D.A. requires that letters be sent to all practicing doctors and that the same procedure should be used for devices like bronchoscopes.

A spokesman for the agency, Brad Stone, said recalls of devices did not require such widespread notification, because devices were in the hands of experts.

"A device in a hospital setting is a narrower-target population," Mr. Stone said. He said the agency was monitoring the Olympus recall and would find out if it was adequate.

Dr. William Schaffner, the chairman of preventive medicine at Vanderbilt who was involved in the inquiry in Tennessee, said the recall had been ineffective, but he also criticized recall procedures.

"Why was this voluntary recall so ineffective?" Dr. Schaffner asked. "Why does the F.D.A. tolerate such an ineffective procedure, which apparently is not monitored in any assiduous way for effectiveness?"