Increased serum levels of alpha1-antichymotrypsin (alpha 1ACT) are observed in some cancer patients, especially those with hepatocellular carcinoma. A possible role of alpha 1ACT in tumour growth has been suggested, but this remains uncertain. We have demonstrated that alpha 1ACT inhibited chymotrypsin-induced apoptosis in rat hepatoma H4 cells. Even low concentrations of chymotrypsin (but not trypsin) induce apoptosis in H4 cells with a minimum effective concentration of 2.4 x 10(-2) units/ml (0.5 microg/ml), and this apoptosis was inhibited by alpha 1ACT in a concentration-dependent manner. Furthermore, the concentrations of alpha 1ACT required to inhibit the apoptosis were lower than normal serum levels. These results may indicate that alpha 1ACT plays a role in the apoptosis of rat hepatoma cells.