Patients with advanced bladder cancer frequently show a low mixed lymphocyte reaction. Depressed mixed lymphocyte reactions may be augmented by removing adherent cells, which appear to be monocytes. Since cancer patients often have a larger proportion of monocytes than normal individuals they may have suppression simply because of an increase in the number of these cells. This possibility was tested by adding increasing numbers of autologous irradiated monocytes or lymphocytes to mixed lymphocyte reactions depleted of adherent cells. The results showed that 1) monocytes from normal individuals increased the mixed lymphocyte reaction at optimal concentrations 3 to 4-fold and suppressed it at higher than optimal concentrations, 2) lymphocytes at equivalent concentrations were not suppressive and 3) monocytes from the cancer patients whose mixed lymphocyte reaction was low did not increase the mixed lymphocyte reaction at any concentration and suppressed it at lower concentrations than monocytes from the normal controls. These results suggest that suppression of the mixed lymphocyte reaction by patient monocytes was not owing to an over-all increase in their number but appeared to reflect a fundamental change in their suppressive function.