Feline alveolar macrophages were inoculated with feline infectious peritonitis virus (FIPV). Cultured macrophages produced substantial amounts of interleukin 1 (IL-1) in the supernatants, as determined by C3H/HeJ mouse thymocyte proliferation assay. FIPV by itself did not affect this IL-1 assay system. IL-1 activity in FIPV-inoculated macrophage cultures was partially reduced by heating at 56°C for 30 min, and completely eliminated by heating at 70°C for 30 min. FIPV-induced feline IL-1 was found to have a molecular weight of approximately 15000 daltons and isoelectric points (pI) of 4.1, 4.8, 5.3 and 6.8, as estimated by Sephacryl S-200 gel filtration and chromatofocusing, respectively. These physicochemical properties of FIPV-induced IL-1 were similar to those of lipopolysaccharide (LPS)-induced IL-1 from feline alveolar macrophages. However, IL-1 with pI 6.8 was more prominent in FIPV-induced IL-1 than in LPS-induced IL-1. Taken together with our previous findings that high levels of IL-1 were produced by peritoneal exudate cells from cats with spontaneous effusive FIP, FIPV infection may trigger the IL-1 production in many kinds of macrophages.