Differentiation-inducing factor (DIF) is a lipophilic hormone of Dicytostelium discoideum and has been shown to exert diverse effects in mammalian cells. We investigated the effect of DIF on cell viability in insulin-secreting INS-1 cells. DIF induced cell death in a dose-dependent manner. In DIF-treated cells, nuclear condensation and shrinkage of the cell body were observed. After 6 h of DIF treatment, cells became Tdt-mediated dUTP-biotin nick end-labeling-positive, and DNA ladder formation was detected, indicating that DIF induced apoptosis in these cells. DIF did not activate caspase-3, a key enzyme mediating apoptotic signals generated by various agents. Furthermore, DIF-induced cell death was not affected by Z-asp-2, 6-dichlorobenzoyloxymethylketone, a broad inhibitor of the caspases. As is the case in other types of cells, DIF increased cytoplasmic free calcium concentration in INS-1 cells. However, DIF-induced cell death was not affected by chelating intracellular free calcium by 1, 2-bis(2-aminoophenoxy)ethane-N, N, N, N-tetra acetic acid (BAPTA). These results indicate that DIF induces apoptosis in INS-1 cells by a mechanism independent of caspase-3. DIF-induced elevation of cytoplasmic calcium does not mediate the effect of DIF on cell death.