Lymphotoxin was found to be present in supernatants from 22 human lymphocytes cultures stimulated with phytohemagglutinin in a dose of 5 and 10 microgram/ml. The lymphocytes were obtained from the peripheral blood of 6 apparently healthy persons. Lymphotoxin activity was determined by simple and objective method, i.e. by staining the target cells (mouse L-cells) monolayer with crystal violet, with the following determination of optic densities of the L-cells lysates at 570 nm in the spectrophotometer. As revealed, 1 : 5 dilutions of the supernatants from the lymphocyte cultures incubated for 48 hours inhibited the L-cells growth by from 40 to 60%. With further incubation of the cultures (up to 72 and 96 hours) the cytotoxicity of their supernatants for the target cells showed no increase, whereas the blasttransformation index reaches the maximal value by 72nd incubation hour. Supernatants from unstimulated lymphocyte cultures failed to produce any cytotoxic effect on L-cells.