The nation's largest early screening program to identify lung cancer in smokers has not turned up any evidence that the death rate from the disease is affected by early detection and treatment, several medical experts connected with the program said yesterday.

The experts called the preliminary findings from the massive screening program of 30,000 people who smoke a pack a day or more "dishearting" and said that even with monitoring as often as three times a year, lung cancer rates among those in the project did not show any significant drop.

The program, which is funded by the National Cancer Istitute, is being conducted at the Mayo Clinic in Rochester, Minn., the Memorial Sloan-Kettering Cancer Center in New York City and Johns Hopkins University Medical School in Baltimore, Md.

Results of the screening program, which is to continue until at least 1981, are being watched by several industries where workers have high lung cancer rates, Johns-Manville, the nation's largest asbestos manufacturer, is already using some of the screening techniques on its employes and federal health officials are using the techniques to monitor uranium miners.

Although none of the program's results has yet been published, a report on the preliminary findings is scheduled for publication soon in the Journal of the American Medical Association.

A separate report is to be distributed next month to staff members of the Mayo Clinic. That report says that the 10,000 volunteers in the clinic's program were divided into two groups. One group received chest X-rays and sputum cytology examinations every four months. The other group was tested upon entering the program and thereafter got annual reminders to follow their normal medical checkup practices for cancer detection.

"There is at present no significant difference in total lung cancer death in either of the two groups and reduction of lung cancer mortality remains the crucial issue," the Mayo report states.

Dr. Robert Fontana, director of the clinic's lung project, said yesterday that the early screening did permit surgical removal of the tumors. "But we have at present no good evidence that periodic screening will reduce mortality for lung cancer," he said.

At Johns Hopkins, Dr. Melvin Tockman said the project was begun because of requests from patients and staff members for an early detection procedure for persons with a ghigh cancer risk. All of the volunteers in the program are men over 45 who smoke at least a pack of cigarettes daily, Tockman said.

The number of lung cancer cases that developed in the study group ranged from 3.7 per 1,000 persons at Johns Hopkins and Sloan-Kettering to about 4.5 per 1,000 at the Mayo Clinic, where the men were slightly older. The cancer incidence rate was about 10 times that of the general population, Tockman said.

Dr. Michael Melamed, head of the Sloan-Kettering study, said one major problem was that the early detection caught certain types of cancer which developed slowly and might be treated at a later stage, but missed a type called "oat cell" cancer until it was beyond treatment. Oat cell cancer spreads rapidly through the body and is usually fatal, he said.

Federal cancer officials said that despite the discouraging preliminary results, they were optimistic that some beneficial efforts of early screening would turn up.

"We are urgin people at this point to go slow on large and costly early screening programs," said Dr. Margaret Sloan, of the National Cancer Institute. "At this point they do not appear to be effective."