A hematological assay is described in which the blood coagulation potential of a body fluid is assessed by reacting a sample of the body fluid with an amount of an activator reagent comprising: (a) a predetermined amount of factor Xa or a hematologically equivalent mutant thereof, and (b) a predetermined amount of factor Va, a hematologically equivalent mutant thereof or an enzyme activating endogenous factor V, (c) (optionally) phospholipids. The reagent may be dry (e.g. lyophilised) or in an aqueous solution preferably buffered to a pH from 6 to 10 (preferably 7 to 8), if desired incubating, if necessary inducing coagulation by the addition of one or more coagulation accelerants such as calcium chloride, and establishing a value indicative of the coagulation potential, e.g. by measuring the time to clotting on an optical coagulometer or through use of a chromogenic substrate. It is preferred to use at (b) factor V activator from purified Russell's Viper venom (RVV-V). An activator reagent is also described containing the components mentioned above preferably in one or more buffer solutions or in dried, e.g. lyophilised form.