The aim of the study was to determine the usefulness of FCoV Ab rapid serological tests in the diagnosis of the effusive form of FIP in cats. The cats included in the study were divided into two groups. The study group consisted of 40 cats with a strain of FCoV causing FIP (the presence of the M1058L mutation) in the abdominal fluid determined using PCR. The control group consisted of 15 cats with ascites caused by factors other than FCoV infection. Serological examination demonstrated the presence of antibodies to feline coronavirus in 28 out of 40 samples of the fluid collected from animals included in the study group, which constituted 70.0% of the tested samples. No antibodies to coronavirus were identified in any of the peritoneal fluid samples collected from the cats included in the control group using rapid immunochromatographic tests. The results obtained in our own studies demonstrated that the serological test ensured very high probability, especially in the detection of infected animals, as well as, although with a slightly lower probability, in the exclusion of the presence of FIP virus infection in the samples of fluid collected from the peritoneal cavity.