Proteins expressed by plasmids encoding human cyclins and cyclin-dependent kinase 2 (CDK2) were used as antigens in immunoblotting. Fifteen of 100 patients with hepatocellular carcinoma (HCC) were found to have autoantibodies reactive with cyclin B1 and with a 40-kd degradation product of cyclin B1-glutathione-S-transferase (GST) fusion protein. Only one serum was found to react with cyclin A and another single serum with CDK2 but no antibodies were detected to cyclin D1 and E. The basis for autoimmune responses to cyclin B1 in HCC are unknown at the present time but the possibilities might include aberrations in cyclin B1 regulation leading to altered product or its expression which resulted in stimulation of immune reactions.