Line 10 hepatocarcinoma cells derived from ascites in a strain 2 guinea pig were tumorigenic when transferred intradermally. After they had been cultured in vitro for 20 days or more in medium enriched with 10% fetal bovine serum (FBS), they became immunogenic. Injections of immunogenic cells did not cause lethal tumors, and recipients were resistant to subsequent challenges with tumorigenic line 10 cells. Resistance was specific since growth of line 1 cells, a syngeneic but antigenically distinct tumor, was not affected. Cells cultured in medium enriched with 10% calf bovine serum or 10% normal guinea pig serum or in reduced concentrations of FBS were less effective in inducing resistance. When cultured line 10 cells were injected ip into normal guinea pigs, ascites tumors developed that were tumorigenic. The growth rate of line 10 cells in culture was considerably decreased as determined by reduced [3H]thymidine incorporation and mitotic indices. The mechanism(s) responsible for enhancement of immunogenicity in cultured line 10 cells is discussed but was not determined.