The histologic pattern of tumor growth at tumor-nontumor boundaries was studied in 60 livers bearing hepatocellular carcinoma. Three growth patterns, arbitrarily described as "sinusoidal," "replacing," and "encapsulated," were distinguished. Cancer cells growing in the sinusoids between liver cell cords (sinusoidal pattern) were anaplastic; those growing in an expansile fashion and acquiring a fibrous capsule (encapsulated pattern) were most differentiated; and those growing into the cord of liver cells and replacing them (replacing pattern) were differentiated to an intermediate degree. There was certain relation between the histologic growth patterns and gross morphologic features of the tumors. Test results for 20 of the 60 cases were positive for serum hepatitis B surface antigen (HBsAg), and the livers also contained orcein-positive cells. Orcein-positive cells were frequently seen at the border between tumor and parenchyma. Cells containing HBsAg as an orcein-positive inclusion were present in cancer tissue in three cases. When serial sections were made from such areas and stained alternately with hematoxylin and eosin and orcein, it was found that these cells were hepatocytes blended with cancer cells. This phenomenon was related to the growth pattern of tumor cells. Orcein-positive cells were never found in metastatic lesions.