The blood coagulation systems were examined in 42 feline immunodeficiency virus- (FIV) seropositive cats measuring the prothrombin time (PT), activated partial thromboplastin time (aPTT), thrombin time (TT) and fibrinogen concentration (FC). In addition, the plasma albumine and total protein concentrations (TPC) were measured in the same cats. In all cats, PT measured with the optimated test (31.95(1.99 sec.) was between the normal ranges, and therefore, there was not any disorder in the exogeneous system of the blood coagulation. Twelve of infected cats (28.6%) showed a pathologic prolonged aPTT (25.6+18.52 sec.), and 15 of cats (35.7%) showed a prolonged TT (23.73+6.98 sec.). FC in 20 cats (54.05%) and TPC in 10 cats (24.4%) were higher than the reference ranges, but the albumine concentration of 11 infected cats (36.6%) was found to be lower than the normal ranges, TPC and FC in the infected cats with a prolonged TT (30.09±1.89 sec.) were found higher than the cats with a normal (19.89+0,38 sec.) TT (p<0.05). The concentration of albumine in the infected cats with a prolonged TT were observed lower than the cats with a normal TT (p<0.05) In conclusion, this study showed that the FIV seropositive cats had no disorder in the exogeneous coagulation system, But, the endogeneous system of the blood coagulation and the fibrin formation were blocked. Disorders in the endogeneous coagulation system may be the result of decreased activities of factors and blocked fibrin formation. Although the FC was found as normal and high, there was a prolonged TT in infected cats. In spite of decreased plasma albumine concentration, the high TPC in the infected cats, especially in cats with prolonged TT may show that the increase of acute phase proteins such as±and±globulins, and gammaglobulins in the form of paraproteins may have disturbed the formation of fibrin from fibrinogen. Disorders in the endogeneus coagulation sys.tem and in the formation of fibrin did not cause hemorrhagica! sign in the infected cats. According to these results, disorders of the blood coagulation in FIV-seropositive cats may not be dangerous for spontaneous bleeding.