Characteristics of the cytotoxicity caused by supernatants of lymphocytes activated in culture are presented. The cytotoxic factor (lymphotoxin, LT) from human spleen cells incubated with phytohemagglutinin in serum-free medium was partially purified by gel electrophoresis. The cells left on the monolayer after incubation with LT demonstrated no detectable abnormalities. Increased permeability of the cell membrane to 86Rb did not occur. Incorporation of 3H-leucine or 3H-thymidine was unimpaired when related to the number of viable residual cells. Cells died and fell off the monolayer even as 3H-thymidine incorporation per cell was unchanged. Kinetic analysis indicates that cell death increases exponentially with time and is second order for the concentration of LT. Cell death is caused by a mechanism other than a general inhibition of protein or DNA synthesis.