Solid-phase immunoglobulin M (IgM) antigen capture enzyme immunoassay (AgCEIA) and antibody capture enzyme immunoassay (AbCEIA) were developed for the diagnosis of BK virus (BKV) infections. Of 37 serum samples from renal allograft recipients, 15 were positive for BKV IgM antibody by either AgCEIA, AbCEIA, or antigen capture radioimmunoassay. False-positive IgM results were observed in the AgCEIA in the presence of high levels of BKV IgG antibody (titers greater than or equal to 1:51,200), when rheumatoid factor (RF) titers were greater than or equal to 1:20, or in the presence of high levels of RF (titers greater than or equal to 1:10,240) when BKV hemagglutination inhibition titers exceeded 1:40. False-positives due to RF could be eliminated by treatment of sera with anti-human IgG antisera or IgG-coated latex particles. The presence of RF did not, however, produce false-positive results in the AbCEIA. Both AgCEIA and AbCEIA were specific for BKV IgM antibody, as 14 serum samples containing either JC papovavirus, cytomegalovirus, rubella virus, hepatitis A virus, or hepatitis B virus core IgM antibody were negative in both EIAs. Comparison of results obtained for 37 serum samples revealed 14 positive by radioimmunoassay and 11 positive by both AgCEIA and AbCEIA. Both EIAs detected BKV IgM antibody in sera of renal allograft patients and patients on renal dialysis who had reactivated BKV infections persisting for several months after transplantation.