By utilizing the fact that heparin and protamine sulfate directly neutralize each other, it is possible to quickly detect excess levels of protamine (anti-heparin activity) by back-titrating a plasma specimen with predetermined amounts of heparin. The approach suggested allows for the simultaneous definition of low levels of heparin, and incorporates an equally rapid and accurate measurement of high heparin concentrations. The methodology presented employs heparin assays using the "Protopath" technic. The concept appears to be applicable to other test systems currently designed to monitor heparin activity.