The hemocytometer leukocyte adherence inhibition technique was used to study cell-mediated immuno-activity of patients with lung cancer. KCl extracts (3.5 M) from the lung cancer cell line Calu-1 and the breast cancer cell line MCF-7 were used as antigens. Of 138 patients with lung cancer, 85% showed a positive response against the Calu-1 antigen. The response was independent of the histological type of the tumor and was the same among untreated patients, patients undergoing different types of treatment and patients who died within 3 months after blood collection. Twenty-five percent of the untreated lung cancer patients also reacted against the breast cancer antigen. Among lung cancer patients undergoing different types of treatment, 36% reacted while 50% of the patients who died within 3 months after blood collection reacted against the breast cancer antigen.