The advent of heparin fractions for clinical use has prompted a reevaluation of the available methods for the assaying of heparin activity in blood. Newly developed heparin fractions are routinely evaluated in terms of their anti-Xa and anti-IIa amidolytic USP, APTT, and anti-Xa coagulant actions. A wide variation between these assays exists due to the use of nonstandard reagents and varying assay conditions. Amidolytic assays, although more biochemically defined than coagulant assays, do not use a natural substrate and work in a diluted plasma system. This may account for discrepancies and differing sensitivities between results in the amidolytic and coagulant assays. The APTT, PTT, and TT may not be effected by the LMFs, and these are the tests currently used to monitor patients on heparin therapy. A poor correlation is observed between the global tests with the newly developed anti-Xa and anti-IIa assays. Furthermore, marked differences in the inhibitory responses are observed with heparin and its fractions. The anti-Xa and anti-IIa assays may provide a more sensitive assay than the APTT, but the poor correlation between assays suggests a multiple effect of heparin and its fractions. Which assay or assays are the best measure of the antithrombotic efficacy of this drug remains to be determined. However, a random selection of an assay is surely not a proper means to monitor these drugs, and a battery of current methodology should be considered.(ABSTRACT TRUNCATED AT 250 WORDS)