The effect of the cytostatic drug Platidiam (Lachema, Brno, Czechoslovakia; effective substance cis-DDP) on the primary antibody response of mouse spleen cells in tissue culture, blastic transformation of spleen cells after PHA stimulation, body and spleen weight, number of peripheral blood leukocytes, lymphocytes, polymorphonuclear leukocytes (PMNL) and reticulocytes was studied. The substance was administered to inbred C3H mice at single doses of 0.83 mg/kg, 2.5 mg/kg, and 3.35 mg/kg i.p. at time intervals of 3, 7, 14, and 21 days before testing. The weight of the mice decreased only from the third day after the dose of 3.35 mg/kg. The spleen weight was not influenced significantly. The peripheral blood leukocyte number increased on the 7th day after cytostatic application, lymphocyte number was less influenced. After cis-DDP application the ratio between lymphocytes and PMNL changed in favor of PMNL. The greatest increase of PMNL was recorded on the 7th day after cis-DDP administration. The number of nucleated spleen cells increased on the 7th day. Suppression of hematopoiesis appeared after all doses in the number of reticulocytes within 3 days, rapid regeneration from the 7th to the 14th day. The primary antibody response increased on the 3rd and 7th days, at further intervals it dropped to 20-70% in relation to the drug dose. The blastic transformation test was increased only on the 14th day after the substance application at the dose of 0.83 and 2.5 mg/kg. The dose 3.35 mg/kg suppressed the cell proliferation at all intervals tested.