Abstract 1. 1. A patient (J. L.) with a coagulation deficiency first reported as idiopathic hypoprothrombinemia and later as a deficiency of stable prothrombin conversion factor, was restudied. Her deficiency appeared to be the same as that defined as Stuart factor deficiency. 2. 2. The patient repeatedly demonstrated a prolonged Quick time, which was corrected by plasma from Owren's patient with proconvertin deficiency but not by serum from Hougie's patient with Stuart factor deficiency. 3. 3. The two-stage prothrombin determination demonstrated a normal amount of prothrombin but a delayed rate of prothrombin conversion. This was corrected by normal plasma, normal serum, and proconvertin deficient plasma, but it was not corrected by normal barium sulfate adsorbed plasma or Stuart factor deficient serum. 4. 4. The patient's abnormal thromboplastin generation test was corrected by normal serum and by hemophilia B serum, but it was not corrected by normal barium sulfate adsorbed plasma or Stuart factor deficient serum. 5. 5. The patient's prolonged Russell viper venom (Stypven) time was corrected by normal plasma and by proconvertin deficient plasma, but, not by Stuart factor deficient serum. 6. 6. In the specific assay for proconvertin, the patient's plasma and Stuart's serum were observed to have normal activity. In a modified specific assay for prothrombin, using Russell viper venom and cephalin as thromboplastin and a serum eluate, the prothrombin activity was normal.