The congenital deficit of factor-X is extremely rare and usually goes with mild bleeding tendency. The variations of the factor-X activities are described individually and depend on the tests used, that is: activating factor-X by tissue or blood thromboplastin, Russel's Viper Venom, by neutralisation of the antibodies or immunoprecipitation. The test depends on the possibility of the specific congenital deficient plasma or on the use of the filtered bovine plasma. That's why it is rather difficult to classify the cases and order the minimum of the hemostatic level. Besides a short description of some physical, chemical and biochemical characteristics of factor-X, present the case with congenital deficit of Prower-Stuart factor discovered in our laboratory, together with the findings and procedures in identifying and confirming it.