Sex-specific elimination of cells by apoptosis plays a role in sex determination in Caenorhabditis elegans. Recently, a mammalian pro-apoptotic protein named F1Aalpha has been identified. F1Aalpha shares extensive homology throughout the entire protein with the C. elegans protein, FEM-1, which is essential for achieving all aspects of the male phenotype in the nematode. In this report, the role of FEM-1 in apoptosis was investigated. Overexpression of FEM-1 induces caspase-dependent apoptosis in mammalian cells. FEM-1 is cleaved in vitro by the C. elegans caspase, CED-3, generating an N-terminal cleavage product that corresponds to the minimal effector domain for apoptosis. Furthermore, CED-4 associates with FEM-1 in vitro and in vivo in mammalian cells and potentiates FEM-1-mediated apoptosis. Similarly, Apaf-1, the mammalian homologue of CED-4 was found to associate with F1Aalpha. These data suggest that FEM-1 and F1Aalpha may mediate apoptosis by communicating directly with the core machinery of apoptosis.