The enzyme-linked immunosorbent assay (ELISA), hemagglutination inhibition test (HAI), and immunofluorescent antibody staining (IFAS) test were compared for their ability to detect IgG antibody to BK virus (BKV). Antibody titers to BKV antigen were determined for 35 sera by each of the three assays. The ELISA tests agreed with either HAI or IFAS with regard to BKV antibody status in all but one serum. On this basis the HAI accounted for four discordant determinations while IFAS was not in agreement in three cases. Geometric mean titers of ELISA and HAI were similar but greater than that of IFAS. The ELISA is particularly well suited for use in large-scale seroepidemiological studies since automated spectrophometric equipment can provide rapid, objective, numerical data.