['We have inquired into the reason why patients with cancer-associated retinopathy (CAR) produce antibody reactions with the 23-kd retinal CAR antigen. Possible reasons include the expression of this antigen in the related carcinoma. Previous studies have failed to identify any antigenic counterpart expressed by in vitro cultivated small-cell carcinoma of the lung. We, therefore, inquired into the effects of in vivo cultivation of the cancer cells and its influence on protein expression, with specific reference to the appearance of the 23-kd retinal CAR antigen.', 'A complementary DNA library was prepared from small-cell carcinoma of the lung cells propagated intraperitoneally in Lewis rats and probed with antibodies reactive with the 23-kd retinal CAR antigen.', 'We found evidence of the expression of a cancer-associated gene in ascites-propagated small-cell carcinoma of the lung that encodes for a protein antigenically similar to the 23-kd retinal CAR antigen. A complementary DNA encoding this protein revealed complete DNA sequence homology with the retinal CAR antigen showing the cancer cells are expressing this photoreceptor protein.'] Does intraperitoneal cultivation of small-cell carcinoma induce expression of the retinal cancer-associated retinopathy antigen?