Researchers have found a chemical that may allow early detection or better monitoring of a virulent form of lung cancer, according to a report in Science magazine.

One-fourth of the nation's several million lung cancer victims incur small-cell cancer of the lung, said Dr. Terry Moody of George Washington University, head of the team that found the link between such cancer and the chemical bombesin. The disease kills 20,000 people annually, he said.

Bombesin was produced in large amounts by small-cell lung cancer grown in the laboratory, Moody said. Other forms of cancer grown in the laboratory contain no detectable amounts of bombesin, according to the report in the Dec. 11 issue of Science.

"We hope that we will be able to detect the presence of the small-cell cancer early by looking for elevated levels of bombesin" in the body, Moody said.

He is now beginning a two-year study here that involves more than 100 patients and is aimed at determining if bombesin is a practical marker for diagnosis of the disease.

Other such chemical markers have been sought for different forms of cancer, mostly without success. The difficulty often stems from doctors' inability to obtain a measurement of the marker chemical that is clearly different than the natural background levels of chemicals.

Some markers, however, have proved useful not in diagnosis, but in giving an indication of how treatment is progressing against a disease.

Moody has gone far enough to show that high and measurably different levels of bombesin do occur in the bodies of patients who have far-advanced cases of small-cell lung cancer.

Among other researchers on the problem are Candace Pert of the National Institutes of Mental Health, and Adi Gazdar, Desmond Carney and John Minna of the National Institutes of Health.