Monoclonal antibodies specific for the envelope (E1), peplomer (E2), and nucleocapsid (N) polypeptides of feline infectious peritonitis virus (FIPV) were used in rapid, competitive enzyme-linked immunosorbent assays (ELISA) to study the humoral immune response of cats to FIPV infection. Results from the competitive ELISAs were correlated with those from immunofluorescent antibody assays (IFAs) on 203 samples obtained from 64 individual cats. The IFA results correlated best with those obtained with the anti-E1 specific competitive ELISA (85.7%). In contrast, anti-N and anti-E2 competitive ELISA results correlated with IFA results only 65.5 and 2.4% of the time, respectively. The results of the anti-E1 specific competitive ELISA were not influenced by the total immunoglobulin concentration or the possible presence of free viral antigens in the serum. These results suggest that a competitive ELISA involving the use of enzyme-conjugated monoclonal antibody to the E1 glycoprotein of FIPV is a simple and rapid replacement for the more cumbersome IFA.