A study of antibodies to human papillomavirus (HPV) 1 is reported in a series of sera from 126 patients with papilloma lesions of different clinical types (verruca plantaris, vulgaris, plana, condyloma acuminata, laryngeal papilloma) using a sensitive solid-phase enzyme-linked immunosorbent assay (ELISA). Our findings showed a significant prevalence of specific HPV 1 IgG antibodies in patients with plantar warts (65%). This incidence was considerably lower in patients with other papilloma lesions, in which positive sera may reflect a past infection with HPV 1. The ELISA test was evaluated as nearly 100 times more sensitive than the indirect immunofluorescence test, and IgG titers to HPV 1 in patients' sera were usually low. Serum samples taken at different times during the course of HPV infection in a few patients with refractory lesions did not show any significant change in their antibody titer.