DNA synthesis in cultures of PHA-stimulated blood mononuclear cells from patients with advanced malignant tumors of the ovary before treatment was significantly reduced and responses of lymphocytes alone were, on an average, 40% lower than in control subjects. Adherent cells were found to suppress the PHA response of autologous lymphocytes from 6/12 patients before treatment, 4/18 patients during chemotherapy and only in 2/19 control subjects. The net effects of adherent cell-lymphocyte interactions in autogeneic vs. allogeneic combinations were frequently discordant, which suggests shifts in lymphocyte subpopulations responsive, respectively, to suppressor- and helper-type mediators released by monocytes in patients. PHA responses of mononuclear blood cells from patients after the 1st and 2nd round of chemotherapy were usually higher than the mean in untreated patients, but declined rapidly after subsequent rounds of cytostatic drug treatment and were deeply impaired in a few cured patients examined many months after therapy. Previous radiotherapy appeared to have little effect on the magnitude and patterns of changes of blood mononuclear response to PHA during subsequent chemotherapy.