The target antigens for the complement-dependent cytotoxic antibodies in D23 tumour-bearer serum from rats carrying the chemically induced D23 hepatoma were extractable from extranuclear membranes of tumours with chloroform/methanol. The antigenic activity, measured by inhibition of D23 TBS cytotoxicity against D23 cells, was recovered in the phospholipid-containing fraction after silica gel chromatography. Preparative thin-layer chromatography (in chloroform/methanol/0.25% CaCl2) revealed that the antigenic activity migrated similarly to phosphatidylethanolamine and cardiolipin, although these phospholipids did not cause the inhibition seen with the antigenic fractions. Phospholipase A2 digestion of the phospholipid fraction did not affect the antigenic activity nor did it after the mobility of the antigen when analysed with thin-layer chromatography. From these results it was concluded that the antigen extracted from D23 hepatoma may be of phospholipid nature, although its molecular identity remains to be established.