The present study was carried out to detect FIP-virus antibodies in feline sera and peritoneal fluids of unknown history and origin. During a five years period a total of 1976 blood samples and exudates from cats were investigated by a recently developed immunofluorescence assay. 43 and 70 percent of sera and punctates were positive for FIP-virus-reacting antibodies. Special emphasis is given to the reliability of the test procedure. It was ascertained by comparing post mortem findings of pathognomonic diseased cats with fluorescence-serological results. The technique used could routinely be applied. It is assumed that the prevalence of FIP-virus infections in our cat population is spreading.