Peripheral blood lymphocytes (PBL) from cancer patients were first tested for the proliferative response to concanavalin A (Con A). The lymphocytes which had lower response to Con A than the normal control were supposed to have relatively greater numbers of putative suppressor cells, or higher suppressive activity was anticipated in the PBL. The PBL were then treated with mitomycin C, added to normal lymphocytes in the presence of Con A, and cocultured for further investigation of the activity of the putative suppressor cells as determined by the effect of these putative cells on the responses of normal lymphocytes to Con A. In many of our studies inconsistent results showed between two types of assay systems. Not all patient's lymphocytes showing depression in response to Con A could suppress the proliferation of normal lymphocytes in response to Con A in coculture systems. However, some of the patients' lymphocytes, despite not showing a depressed response to Con A in the direct assay, were able to inhibit the response of normal lymphocytes to Con A in coculture. The contradictory data imply that it is inappropriate to conclude that suppressor cells are present at elevated levels in cancer patients by relying solely on the evidence of a depressed response to mitogens, either in a direct stimulation assay or in a coculture system. Our results possibly reflect that the development of cancer is not directly linked to the elevation of suppressor cell activity. Other more complicated mechanisms may be involved.