Immunohistochemical localization of P-450 HFLa, a form of cytochrome P-450 in human fetal livers was investigated in human hepatocellular carcinoma. The cytoplasm of carcinoma cells positively reacted with anti-P-450 HFLa antibodies. It was found that the carcinoma cells showing a pseudoglandular pattern or poorly differentiated appearance exhibited a weaker reactivity with anti-P-450 HFLa antibodies than did relatively differentiated carcinoma cells. In the case of hepatoblastoma, the polygonal or round-shaped tumor cells which differentiated into epithelial structure exhibited a positive reaction with anti-P-450 HFLa antibodies, whereas the spindle-shaped tumor cells which showed a sarcomatous appearance did not react with the antibodies.