Haematology, antibody titers and serum protein electrophoresis from 48 cats (34 effusive and 14 noneffusive forms) affected with feline infectious peritonitis (FIP) were studied and compared with those of 20 healthy cats. In the effusive form, antibody titers and protein electrophoresis in the effusions were analyzed. The distribution of the immune cells and of the virus in FIP lesions were also investigated immunohistochemically with the avidin-biotin complex (ABC) method, using antibodies against the FIP virus (FIPV), myelomonocytic (MAC387) and lymphoid (CD3, CD4 and CD8 for T-cells and IgM and IgG for B-cells) antigens. Seropositive animals (antibody titer>1:100) were present among both the FIP infected cats (73%) and the healthy cats (70%). Cats with effusive FIP had neutrophilic leukocytosis (P>0.05), lymphopenia (P<0.01) and eosinopenia (P<0.001). In both effusive and noneffusive forms decreased albumin/globulin ratio (P<0.001) with hypoalbuminemia (P<0.001), hyperglobulinemia (P<0.001) and increased alpha2- (P<0.05), beta- (P<0.05) and gamma-globulins (P<0.001) were found. Hypergammaglobulinemia was not related to the antibody titers, suggesting the presence of other proteins with gamma-motility (e.g. complement fractions). The electrophoretic pattern of the effusions was always similar to that of the corresponding serum. Antibody titers higher than those of the corresponding serum were often detected in the effusions. Immunohistochemical findings were not related to the antibody titers, but they were related to the histological aspect of the lesions. In cellular foci of FIP lesions many virus-infected macrophages and few lymphocytes, mainly CD4+, were found. Extracellular viral and myelomonocytic antigens were also detectable in the foci with intercellular necrosis. Only few FIPV-infected cells were present at the periphery of the larger necrotic foci: in these lesions MAC387+ cells were mainly neutrophils, with many MAC387 macrophages, probably due to their activated state; a small number of lymphocytes, with an increasing percentage of CD8+ cells was present. Lymphocytes were more abundant when cellular foci and FIP-infected macrophages were centered around neoformed vessels. IgM and IgG exposing B-cells were always few and scattered. In conclusion the simultaneous analysis of body fluids and of the cellular composition of the lesions showed a complex immune status, on which type III and type IV hypersensitivity could coexist.