The antibody-dependent cell mediated cytotoxicity of peripheral blood mononuclear cells from 92 patients with germinal cell tumours and 60 healthy male controls was measured against 0, Rh(D) positive human red blood cells sensitized with anti-D antibody. To determine the maximal K-cell activity the enzym-like kinetic model of citotoxicity was employed in which maximal activity was measured in presence of target-cell excess. To avoid variation due to the individual sensitivity of target erythrocytes red blood cells were obtained from a single donor. It was demonstrated that compared to the control group the K-cell activity of patients with germinal cell tumours was significantly enhanced. Cytotoxic activity of patients with clinically detectable tumours was significantly higher than that of patients with no detectable tumour. The K-cell activity of patients with detectable tumours was significantly increased after chemotherapy.