A simple one-stage test for the determination of the Stuart-Prower factor is described. The method is a modification of the classical Factor VII complex assay, using a Russell’s viper venom (Stypven)-cephalin reagent instead of brain thromboplastin. The optimal conditions for the assay system have been systematically investigated. A high dilution of RVV (1 : 200 000) has given the best results. The system is not influenced by different concentrations of fibrinogen, prothrombin, Factor V, Factor VII and plasma thromboplastin precursors other than Stuart-Prower factor. Variations of lipoid and/or platelet contents of the plasma do not alter the results. Stuart-Prower factor is not activated by glass contact. The method has proved most valuable in the detection of heterozygous Stuart-Prower factor deficiencies. Comparison with a specific Stuart-Prower factor assay using patient’s plasma deficient in Stuart-Prower factor gave variations not greater than 15%. If determined simultaneously with the Quick one-stage ‘prothrombin time’, the test may provide a better control of patients under treatment with dicoumarol derivatives. * This study was aided by grants from the ‘Emil Barell-Foundation for Medical Research.’ ** With the technical assistance of Miss D. Gerber.