The ABH blood group antigens were studied in resected specimens from patients with lung cancer employing monoclonal antibodies against A, B and H antigens by the immunoperoxidase technique. There were no differences between paraffin-processed and fresh-frozen materials in detectability of ABH antigens. Comparing specimens at operation with those at autopsy from the same patient, the loss of A and B antigens in the latter was demonstrated more frequently than H antigen. Concerning the relation between the prognoses and the expression of ABH antigens, the group with good prognosis preserved more ABH antigens than that with poor prognosis. In contrast, the group with poor prognosis showed less ABH antigens. There were also statistical differences in antigen detectability and prognosis, particularly in stage 1 patients, adenocarcinomas and well-differentiated lung cancers.