PAP technique and rabbit anti-X serum were used to detect the X protein in tumor and nontumor liver tissues from 34 patients with HCC. The positive rate of the X protein in both tissues were 94.1% and 84.4% respectively. Of the 34 patients with HCC, 27 were complicated by liver cirrhosis, in whom 92.6% were X protein positive in liver cells. It was found that almost all of the liver cells adjacent to the tumor tissue showed strong positive staining. The high frequency and predominant expression of X protein in HCC and liver cirrhosis tissues indicated that X protein may play an important role in hepatocarcinogenesis. X protein was detected in 17.2% of the patients with CAH, which suggested the risk of transformation from CAH to cirrhosis and/or HCC. X protein was first found in bile duct epithelial cells in 59.4% of the patients with HCC, and 6 of 34 HCC were combined with bile duct carcinoma, and some cancer cells were found positive for X protein. It seems that X protein may also be a potential factor in the oncogenesis of bile duct carcinoma.