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Plasma membranes were isolated from rat and mouse livers, one rat hepatoma (and its subline) and two mouse hepatomas, and their lipid class compositions were determined. Lipids accounted for 30 to 35% of the dry weight of the membranes of livers and mouse hepatomas, and for 45% in the case of rat hepatoma-subline. Of the total lipids of rat-liver plasma membranes, 60% consisted of phospholipids, the corresponding values for mouse-liver and rat-hepatoma plasma membranes amounting to 55% and for both mouse-hepatoma plasma membranes to about 50%. The free cholesterol and cholesteryl ester contents of all hepatoma plasma membranes were significantly increased as compared with normal. Evidence is presented that the increase of free cholesterol was not a preparative artefact. The major phospholipid classes in all plasma membranes were phosphatidyl choline, sphingomyelin, phosphatidyl ethanolamine and phosphatidyl serine. The relative proportions in each plasma membrane species could differ appreciably, the mouse- and rat-liver membranes showing the closest resemblance. Possible reasons for (a) the higher level of phosphatidyl serine as compared with published values, and (b) the wide divergencies which may be found among the phospholipid profiles of rat-liver plasma membranes reported by other authors, are presented. Cardiolipin was absent from liver plasma membranes, but some could be found in the hepatoma membranes due to mitochondrial contamination. No consistent phospholipid profile characterized hepatoma as distinct from liver plasma membranes, nor did the hepatoma data-including plasmalogens-resemble the few available data on other hepatomas.

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