A reverse transcriptase polymerase chain reaction (RT-PCR) for the detection of feline coronavirus (FCoV) messenger RNA in peripheral blood mononuclear cells (PBMCs) is described. The assay is evaluated as a diagnostic test for feline infectious peritonitis (FIP). It is based on a well-documented key event in the development of FIP: the replication of virulent FCoV mutants in monocytes/macrophages. To detect most feline coronavirus field strains, the test was designed to amplify subgenomic mRNA of the highly conserved M gene. The test was applied to 1075 feline blood samples (424 from healthy, 651 from sick cats suspected of FIP) and returned 46% of the diseased cats as positive for feline coronavirus mRNA in their peripheral blood cells; of the healthy cats, 5% tested positive. Of a group of 81 animals in which FIP had been confirmed by post-mortem examination, 75 (93%) tested positive, whereas 17 cats with different pathologies (non-FIP cases) all tested negative. In view of the low rate of false-positive results (high specificity) the mRNA RT-PCR may be a valuable addition to the diagnostic arsenal for FIP.