Forty-five sera from men with bladder cancer were examined in a micro solid-phase enzyme-linked immunosorbent assay (ELISA) and in a Western-blotting (WB) assay for the presence of IgG antibodies to papillomavirus (PV) genus-antigens of bovine origin. The ELISA detected PV antibodies in 75.6% of cancer patients. This antibody frequency was significantly higher than that found in both healthy males (22.7%) and patients with urological disorders (24%). A similar correlation among the PV antibody frequencies of the three groups was found with WB assay: 60% of the neoplastic group showed PV antibodies versus 17.3% in healthy males and 32.6% in non-neoplastic patients. Within the same group, 78% to 87% sera showed the same reactivity to both assays. Of these concordant sera, PV positive sera were 55.6% in cancer patients, 13.3% in healthy adults and 19.6% in patients with urological disorders. ELISA PV antibody level in the cancer group was higher than in each of the two control groups. The meaning of the humoral response to PV genus-antigens in men with bladder cancer is discussed.