Abstract Whole human blood was treated with varying concentrations of ethylenediaminetetraacetic acid to determine the effect upon prothrombin consumption. The prothrombin consumption test was combined with the Dreskin two-stage prothrombin test to determine residual prothrombin as a function of incubation time. Ethylenediaminetetraacetic acid in high concentrations inhibited prothrombin consumption; in one concentration prothrombin consumption was accelerated, and in the lower concentrations no effect was observed. These data indicate that EDTA has a diphasic action upon the utilization of prothrombin as a part of its diphasic action in whole blood coagulation.