Feline infectious peritonitis is a feline disease, the causative agent of which is a mutant coronavirus, which leads to the death of young animals. Presently, this pathology is considered incurable, and therefore it requires a detailed study. The purpose of this study was to establish the features of clinical manifestation and haematological parameters in cats with infectious peritonitis. This paper presents the results of a clinical and laboratory study of 12 cats aged from 6 months to 3 years who were diagnosed with effusive and dry infectious peritonitis. Clinical, laboratory, and visual research methods were used. Laboratory analysis included a study of morphological and biochemical parameters of blood, cytological examination of effusion from the abdominal and pleural cavities, and a Rivalta test. As a result of cytological examination of effusion from these cavities, a high concentration of cells, macrophage accumulation, neutrophil phagocytosis, and red blood cells were detected in the entire field of view. It was found that the predictive value of the Rivalta test for feline infectious peritonitis is 62.5%. According to haematological examination, all sick animals were diagnosed with anaemia, lymphopenia, hypoalbuminemia, hyperglobulinemia, hyperbilirubinemia, increased activity of relative liver- specific enzymes (aspartate aminotransferase, alkaline phosphatase). Trobmocytopenia was observed in two animals, and an increase in blood markers of the functional state of the kidneys (creatinine, urea) was found in one cat. The serum albumin/globulin ratio should also be considered an important diagnostic indicator. Thus, with feline infectious peritonitis, this indicator should be < 0.4. In 11 sick cats, this ratio was characterized by values below 0.4, and in one animal this indicator was 0.46. In general, the obtained data of morphological and biochemical blood tests are not specific for feline infectious peritonitis, and therefore the authors of this paper recommend their comprehensive investigation for diagnostic purposes. An objective assessment of the available methods of laboratory diagnostics will contribute to the creation of a diagnostic protocol for feline infectious peritonitis