Sixteen compounds were tested for their ability to inhibit the replication in vitro of feline infectious peritonitis virus (FIPV), a coronavirus that causes a lethal, immunologically mediated illness in domestic and exotic cats. Six of the compounds, when incubated with cells and titrations of the virus, were found to reduce the virus titres by 0.401 to 0.833 log10 (P < 0.05), using the cytopathic effect as endpoint. Further inhibition studies were performed to determine the 50 per cent effective dose (ED50) levels for these six compounds. Selectivity indices (50 per cent cytotoxic dose [CD50]/ED50) provided the following order of antiviral activity: pyrazofuin > 6-azauridine > 3-deazaguanosine > hygromycin B > fusidic acid > dipyridamole. Compounds which had no statistically significant effect on FIPV in the same assay were caffeic acid, carbodine, 3-deazauridine, 5-fluoroorotic acid, 5-fluorouracil, D(+)glucosamine, indomethacin, D-penicillamine, rhodamine and taurine.