Between 1980 and 1987 about 10% of the cats which underwent a post mortem examination at the Institute of Veterinary Pathology of the Freie Universität Berlin were infected with feline infectious peritonitis (FIP). The exudative, granulomatous or mixed form of the FIP are all symptoms of the same disease whose clinical picture is dependent on the state of the cellular immunity. Statistically there is no significant relationship between the form of the FIP and the age of the cats. A breed or a sex disposition is also not apparent. 42 organs and/or tissue samples were taken from a total of 30 cats and were examined both histologically and immunohistochemically. The antigen is located, above all, in the mesothelium and the cells of the mononuclear phagocyte system, whereas the parenchymatous inflammation foci show little evidence of the FIP-virus-antigen. The antigen is, however, also found in the nerve cells. Within the framework of a second viraemia, occurring after infection of the mesothelium (polyserositis), a perivasculitis non purulenta generalisata occurs. Possible excretory organs of the antigen are the respiratory, digestive and urinary tract.