Enhancement of feline infectious peritonitis virus (FIPV) infection of feline macrophages was studied using monoclonal antibodies (MAbs) to the FIPV strain 79-1146. Adherent cells recovered from the feline lung and peritoneal cavity phagocytosed fixed red blood cells, and formed Fc-mediated rosettes. Enhancement of virus infection by MAb was investigated by inoculating alveolar macrophages with a mixtures of viral suspension and MAb, and examining the cells for intracellular viral antigen by the immunofluorescence assay and the amount of infectious virus in the supernatant fluid after incubation. The replication of FIPV in macrophages was enhanced by non-neutralizing MAbs recognizing peplomer protein (S) and transmembrane protein (M) of the virus. Even among the MAbs having the ability to neutralize FIPV strain 79-1146, some reversely enhanced virus infection when they were diluted. The enhancement was suppressed by pretreatment of the MAb with protein A. The enhancement was reduced by the use of F(ab')2 fragment of MAb. These results demonstrated antibody-dependent enhancement (ADE) of FIPV infection in macrophage. The replication of FIPV 79-1146 strain in macrophages from FIPV antibody-positive cats was more enhanced than in those from antibody-negative cats.