Twenty-four hepatocellular carcinoma (HCC) patients, 35 asymptomatic hepatitis B surface antigen (HBsAg) carriers and 30 normal healthy individuals, were included in this study to evaluate the natural killer (NK) cell population and NK cell activity, particularly before and after surgery for HCC patients. The NK cell population was stained with fluorescent labelled monoclonal antibody (leu 11b) and counted under fluorescent microscope. The NK cell activity was evaluated by Cr51 release assay and K562 cells line serving as target cells. The result indicated that there is no significant difference in NK cell population in HCC patients as compared with either the normal control or asymptomatic HBsAg carriers. (p greater than 0.05) However, significantly depressed NK cell activity was observed in asymptomatic HBsAg carriers and HCC patients as compared with that of the normal control. The NK cell activity of HCC patients was significantly depressed further after surgical removal of the tumor on the 1st postoperative week. Though the NK cell activity showed signs of recovery on the 2nd and the 4th postoperative week, it still did not return to the preoperative level even on the 4th postoperative week.