Serum antibodies to the feline infectious peritonitis (FIP) virus were measured in cats, using an indirect fluorescent antibody procedure. Antibody titers of 1:400 to 1:25,600 were seen in cats with both effusive and noneffusive forms of FIP. About 87% of the normal cats in FIP problem catteries and 20% of the cats in the Davis, Ca, area also had antibody titers to the FIP virus, ranging from 1:25 to 1:400. Although the infection rate is high among cats, relatively few of the cats infected with the FIP virus ever develop clinically apparent FIP.