Antibody-dependent enhancement of virus infection is a process whereby virus-antibody complexes initiate infection of cells via Fc receptor-mediated endocytosis. We sought to investigate antibody-dependent enhancement of feline infectious peritonitis virus infection of primary feline peritoneal macrophages in vitro. Enhancement of infection was assessed, after indirect immunofluorescent-antibody labelling of infected cells, by determining the ratio between the number of cells infected in the presence and absence of virus-specific antibody. Infection enhancement was initially demonstrated by using heat-inactivated, virus-specific feline antiserum. Functional compatibility between murine immunoglobulin molecules and feline Fc receptors was demonstrated by using murine anti-sheep erythrocyte serum and an antibody-coated sheep erythrocyte phagocytosis assay. Thirty-seven murine monoclonal antibodies specific for the nucleocapsid, membrane, or spike proteins of feline infectious peritonitis virus or transmissible gastroenteritis virus were assayed for their ability to enhance the infectivity of feline infectious peritonitis virus. Infection enhancement was mediated by a subset of spike protein-specific monoclonal antibodies. A distinct correlation was seen between the ability of a monoclonal antibody to cause virus neutralization in a routine cell culture neutralization assay and its ability to mediate infection enhancement of macrophages. Infection enhancement was shown to be Fc receptor mediated by blockade of antibody-Fc receptor interaction using staphylococcal protein A. Our results are consistent with the hypothesis that antibody-dependent enhancement of feline infectious peritonitis virus infectivity is mediated by antibody directed against specific sites on the spike protein.