An improved one-stage method for accurate assays of factor VIII combines highly reproducible end points with elimination of temporal drift and of subjective factors involved in graphic analysis. Activated deficient plasma substrate (ADPS) is used as a single reagent for parallel tests. The assays can be performed manually or, with much greater precision, on an 8-channel coagulation meter of new design in which end points are recorded automatically and depend on an abrupt clearing of agitated cloudy suspensions. The coagulation time readings are reproducible to 0.5%. Factor VIII levels are read off a standard table or computed from a general equation which is readily programmable on a pocket calculator and is valid over a wide range of concentrations, including very low plasma levels.