SDS-polyacrylamide gel electrophoresis of the nuclear matrix proteins showed a predominance of high molecular weight and low molecular weight polypeptides in the nuclear matrices of hepatomas and cultured Chinese hamster fibroblasts as compared to quiescent and regenerating rat liver. These features were more prominent in solid hepatoma 27 than in Zajdela ascites hepatoma or in cultured cells. In proliferating cells (tumors, regenerating liver, log phase cell culture) a polypeptide band of 150 kD and lamin B were conspicuous at the expense of lamins A and C.