Proliferative activity and lipid composition (phospholipids and gangliosides) were studied in rat hepatoma-27 transplanted subcutaneously or intrahepatically (as models for primary and metastasizing tumors). The mitotic index of subcutaneously transplanted hepatoma far exceeded that of the intrahepatically transplanted tumor. The overall amounts of both phospholipids and gangliosides increased appreciably in the subcutaneously growing hepatoma (in contrast to the intrahepatically growing tumor) in comparison to the control hepatic tissue. The ganglioside composition in the tumors differs from that in the liver: ganglioside GD3 appears, whereas gangliosides GD1b and GT1b decrease in amount in the intrahepatic tumor compared to the control liver and disappear in the subcutaneously transplanted hepatoma. In both tumor types, the amounts of both phosphatidylethanolamine and sphingomyelin exceed the control values. Comparison of these results with previously reported data concerning the phospholipid and ganglioside composition in the regenerating rat liver indicates that the difference in the lipid composition between the subcutaneously and intrahepatically growing hepatomas-27 is due to their different proliferative status and also their microenvironment.