Female adult BUF rats (6-8 week) received sham operation (Sham), 70% hepatectomy (PH); Sham or PH with blood transfusion (BT or PH+BT). BUF 7316A hepatoma cells were inoculated subcutaneously in the neck of rats on the operation day. Tumor size was measured from day 7 to 20 after inoculation. Sera and splenic adherent cells harvested on day 5 from Sham and PH rats were added into mixed lymphocyte cultures (MLR). The result showed that tumor growth in PH or BT rats was significantly promoted as compared to that in Sham rats (P < 0.01, P < 0.05). The most marked enhancement of tumor growth was observed in PH+BT rats (P < 0.001). Sera and splenic adherent cells from PH rat significantly inhibited MLR (P < 0.05). These results suggest that partial hepatectomy and blood transfusion are responsible for the enhancement of tumor growth. Some immunosupperassive factor might be produced in the process of liver regeneration, and blood transfusion might have an additive immunosuppressive effect.