A presumptive case of feline infectious peritonitis (FIP) will be presented with an emphasis on diagnostics. FIP occurs as a result of a mutation of feline enteric coronavirus to a form that can replicate within macrophages. The disease that results is a function of the host?s immune-mediated response to the mutated virus. FIP is debilitating and fatal, therefore accurate and timely diagnosis would be ideal. Diagnosis is difficult because clinical signs are not specific and clincopathologic findings are not pathognomonic. Serology is an important diagnostic tool, but proper interpretation is paramount. An understanding of the diagnostics available is essential in order to utilize those that are most appropriate for the patient and the client.