Two tests, the activated coagulation time test (ACT), and the quantitative protamine titration test (QPT), were examined in detail as representative of a large number of tests potentially useful in determining dose of heparin needed during cardiopulmonary bypass and the dose of protamine needed for reversal of heparin. The variability introduced by the test methods (ACT 6 percent, QPT 8 per cent) was insignificant comparen (greater than 25 per cent) and the variation in plasma volume (14 per cent). Both of these variables affected not only QPT but also any modification of it that measures the level of heparin by titration with protamine solutions. Tests that measure the effect of heparin on the clotting time, of which the ACT is an example, were unaffected by either population variable when used in conjunction with a simply constructed dose-response curve.