The effects of genistein on murine thymocytes for inducing apoptotic cell death and phagocytic activity of peritoneal macrophage were studied in vitro. Addition of genistein (10 and 50$mu$M) to cultured thymocytes from BALB/c mice definitely promoted DNA fragmentation. Also, cytofluorometric analysis of these cells demonstrated a reduction in mitochondrial transmembrane potential ($Delta$Ψm). But, repeated administration of genistein (1 mg/mouse/day) to mice for 7 days did not cause any detectable DNA fragmentation. Genistein decreased lucigenin chemiluminescence and engulfment of fluorescein-conjugated E. coli particles in peritoneal macrophage. These results suggest that genistein induce an apoptosis of thymocyte via reduction in $Delta$Ψm and decrease phagocytic activity of peritoneal macrophage in vitro.