The differentiation-inducing factor-1 (DIF-1) is a signal molecule that induces stalk cell formation in the cellular slime mold Dictyostelium dis-coideum, while DIF-1 and its analogs have been shown to possess anti-proliferative activity in vitro in mammalian tumor cells. In the present study, we investigated the effects of DIF-1 and its analogs on normal (nontransformed) mammalian cells.