Fas is a type I membrane protein and its activation by binding of the Fas ligand or an agonistic anti-Fas antibody induces apoptosis in Fas-bearing cells. In this report we prepared lysates from cells treated with anti-Fas antibody. The lysates induced apoptotic morphological changes in nuclei from normal mouse liver, accompanied by DNA degradation. The apoptosis-inducing activity was quickly generated in cells by anti-Fas antibody and was found in the soluble cytosolic fraction. Induction of the activity in cells was inhibited by a tetrapeptide, acetyl-Tyr-Val-Ala-Asp-chloromethylketone, a specific inhibitor of interleukin-1 beta converting enzyme. Addition of COS cell lysates containing Bcl-2 to the assay significantly inhibited the apoptotic process, indicating that the in vitro process reflected apoptosis that occurs in intact cells.