Six adult specific-pathogen-free cats were inoculated intraperitoneally with a cell culture-adapted strain of feline infectious peritonitis virus. Plasma samples were evaluated for antithrombin-III (AT-III) activities at post-inoculation days (PID) 0, 4, and 11 and at termination on PID 16 (1 cat) or 21 (5 cats). Other hemostatic values evaluated were activated partial thromboplastin times, prothrombin times, thrombin times, fibrinogen, platelet counts, and fibrin/fibrinogen degradation products. Antithrombin-III activity remained within normal or above normal range (89 to 246%) in all cats, with the exception of one cat on PID 4 (AT-III, 70%). Mean baseline AT-III activity for 6 cats at PID 0 was 123%. Mean AT-III activity on PID 4, 11, and 16 or 21 was 98, 162, and 130%, respectively. On PID 4 and 16 or 21, results of coagulation screening tests indicated that all cats had disseminated intravascular coagulation. Histologically, cats also had severe fibrinonecrotizing thrombovasculitis.