A rapid, specific, one-stage assay for prothrombin (SIMA assay) is described. It uses a commercial thromboplastin enriched with bovine factors V and fibrinogen and easily made aged serum. The coefficient of correlation with the specific one-stage assay of Owren is 0.93. It is suggested as a valuable adjunct to the prothrombin time for monitoring patients taking oral anticoagulants. Its rapidity makes it of value in assessing patients with suspected or established consumption coagulopathies. In evaluation of patients with hepatocellular disease the SIMA assay may be preferable to two-stage methods, since it is virtually unaffected by changes in antithrombin-III.