To study the effect of hepatectomy on the growth of liver tumor, Shionogi Carcinoma 42, a mammary tumor, was transplanted into the liver of mice which had undergone 40% hepatectomy. The liver tumor and the number of pulmonary metastases in hepatectomized mice were significantly larger than those in nonhepatectomized mice. Responses to lectins and IL-2, subpopulations, and cytotoxicity to YAC-1 and P815 cells of splenocytes were assessed to evaluate immunological status. At the initial phase after hepatectomy and tumor transplantation into the remaining liver, NK activity transiently increased, and function of B and T cells, especially of helper T cells, decreased, while B-cell function recovered beyond normal levels in a later phase. These results suggest that liver may play an important immunological role and that the immunological modification after hepatectomy may be responsible for the accelerated growth of liver tumor. Accordingly, some adjuvant immunotherapy may be recommended for the prevention of recurrence after hepatectomy for liver tumor.