UFH and its derivatives, LMWHs, are effective and safe in the prevention and treatment of DVT. The mechanism for this effectiveness has been difficult to explain, given that the anti-IIa activity of LMWHs is important for their antithrombotic activity and the reported half-life of the anti-IIa activity of LMWHs is very short. The standard chromogenic anti-IIa assay is performed in an artificial system consisting of highly diluted plasma to which antithrombin III is added. It is possible, therefore, that the apparently short half-life of the anti-IIa activity is dependent on the limitations of the anti-IIa assay, commonly used in the pharmacokinetic studies. We have developed an anti-IIa assay that is more sensitive than the standard one. Our assay is based on the ability of UFH or LMWHs to catalyze the formation of TAT complexes. PTNA was able to detect in vitro the anti-IIa activity produced by 0.01 anti-Xa IU/mL of UFH or 0.05 anti-Xa IU/mL of LMWHs. Ex vivo, it was able to describe the time course of plasma anti-IIa activity of very low doses of UFH (intravenous or subcutaneous) or LMWHs. To characterize better the role of the anti-IIa activity of LMWHs, the pharmacokinetic properties of two of these agents have been evaluated in humans, assessing the anti-IIa activity by PTNA. Fraxiparine, 7500 and 10,000 ICU, and Enoxaparine, 20 and 40 mg, were administered subcutaneously to six healthy volunteers.(ABSTRACT TRUNCATED AT 250 WORDS)