Apoptosis is a form of cell death in which the cell "participates," such that metabolic energy and often protein synthesis are required for the death to occur. Once begun, the process of apoptosis proceeds in an ordered fashion. In the earliest phase DNA fragmentation occurs, accompanied by cell shrinkage and dilation of the endoplasmic reticulum. This is followed by cell fragmentation with the formation of sealed membrane vesicles, termed apoptotic bodies. In the present study we have demonstrated that the fungal metabolite cytochalasin B inhibits cell fragmentation and the formation of apoptotic bodies, probably by its ability to interfere with actin polymerization. This effect was seen when HL-60 cells were pretreated with cytochalasin B and then exposed to one of a number of apoptosis-inducing agents, including UV irradiation, camptothecin, aphidocholin, or PMA plus ionomycin. The observed effect was not peculiar to HL-60 cells, inasmuch as it was also seen for both Molt-4 and U-937 cell lines. Cytochalasin B had no effect on DNA fragmentation occurring in the earliest stage of apoptosis, and it appeared to have no inhibitory effects on nuclear fragmentation. Staurosporin had an effect similar to that seen with cytochalasin B, probably due to its ability to inhibit protein kinase C, which is a known potentiator of microfilament assembly. These data demonstrate that microfilament assembly is necessary for the formation of apoptotic bodies in the later stages of the apoptotic process.