Abstract: A series of studies were made on factors influencing the coagulation time of rabbit's oxalated plasma in the biological assay of thtomboplastin. It was found that the anticoagulanting action of sodium Chloride as reported by Jaques and Dunlop (1945) could be greatly reduced in the presence of thromboplastin and the Change of the concentration of calcium chloride exerted least effect on the coagulation time, while the concentration of thtomboplastin in the plasma mixture adopted was 0.045%. It was shown that a low concentration of calcium chloride such as 0.003—0.009%in the mixture and a temperature of 25℃ adopted during the assay gave favourable results with greater accuray.It was suggested that while the assay proceeded under the above-mentioned conditions,the coagulation time of the plasma in the blank tube should be within the range of 17—23 minutes and that in the specimen tube should not be more than 6 minutes. Following the modified procedure, over thirty commercial samples of thromboplastin were tested. An analysis of the results proves that the improved method is able to detect samples of poor quality or samples deteriorated during storage.