Stimulation of rat lymphocytes by allogeneic and xenogeneic (mouse) embryo fibroblast monolayers or by pokeweed mitogen (PWM) generate two different cytotoxic manifestations: a) cell-mediated target cell lysis obtained by contact between effector cells and target cells; b) cytotoxicity caused by lymphotoxin (here termed "necrosis"). Activation of hypersensitive lymphocytes by soluble antigen, e.g., keyhole limpet hemocyanin (KLH), produces only necrosis but no lytic effect. This difference between mitogen and antigen activation is attributed to the affinity of the mitogens for target cell membranes. There is a marked difference in the time of onset of the two phenomena. Ten minutes after the effector cells have been introduced to the target monolayers, lysis of fibroblasts becomes visible, whereas necrosis usually develops only 48 to 72 h later. A clear difference in the morphology of the two types of target cell killing was seen when the cultures were stained in situ with trypan blue. Lysed target cells produced blebs and underwent fragmentation, scattering cytoplasmic droplets. During a 10-min to 24-h period, these cells excluded the dye. Only much later (after greater than 30 h), when the cell remnants had undergone a further slow "decay" process, did they stain with trypan blue. In cultures rendered necrotic by lymphotoxin, the killed fibroblasts preserved their morphological integrity but the nuclei became markedly stained with trypan blue. Lymphotoxin was found to be effective only in highly concentrated macrophage and fibroblast cultures. In sparsely populated macrophage cultures, the mediator(s) did not effect cell death but nonetheless exerted marked morphological changes. Vacuoles containing lipids developed and eventually occupied the greater volume of the cytoplasm. Giant lipid droplets were seen leaking out of the macrophage cytoplasm. It is suggested that lymphotoxin does not kill directly, but rather that metabolic products released by the vigorously stimulated target cells cause the necrotic effect in the cultures.