Signaling via the CD3/T cell receptor complex induces programmed cell death (apoptosis) in IL-2-dependent human T lymphocytes. We have investigated the time course kinetics of the induction of programmed cell death in cloned CD4+ T cells. Morphological changes (cell shrinkage) were noted by flow cytometry as early as 3 hr after stimulation of clone cells with ionomycin, PHA, or anti-T cell receptor antibody BMA 031. Fragmentation of DNA became visible 3 hr after ionomycin stimulation, and 5 hr after PHA or BMA 031 stimulation, and peaked after 8 to 24 hr. Significant cell death (as revealed by flow cytometry analysis of propidium iodide-positive cells) was detected 5 hr after ionomycin treatment and 10 hr after PHA or BMA 031 treatment. With all three stimuli, maximal cell death was recorded after 16 to 18 hr. Taken together, our data indicate that the activation-induced death of mature human T cells is a rapid event which is completed within 18 hr. Induction of DNA fragmentation is preceded by the reduction of cell size which can be readily examined by flow cytometry.