The effects of mast cells (MCs) isolated from rat peritoneal cavity on rat hepatoma cell line (CBRII7919) in vitro were studied with phase contrast microscope, scanning and transmission electron microscope. The results showed that different degrees of degeneration were presented in all CBRH7919 cells and a few of them exhibited necrosis or disruption when CBRH7919 cells were cocultured with MCs for 24 h. In situ hybridization demonstrated that the expression of c-myc mRNA in CBRH7919 cells was markedly reduced by MCs. These results suggest that MCs had an antitumor effect.