In a double-blind, randomized, cross-over study the effects of single subcutaneous doses of 120 anti-Xa units/kg body wt. of three different low molecular weight heparin (LMWH) preparations were investigated in 15 healthy subjects by determination of thrombin-antithrombin III complex (TAT), prothrombin fragment 1.2 (f1.2), and beta-thromboglobin (beta-TG) in shed blood and in venous blood. Certoparin, dalteparin, and enoxaparin significantly inhibited coagulation activation marker formation in shed blood. The substantial inhibition of TAT and f1.2 formation was slightly more pronounced in response to certoparin. beta-TG was decreased following certoparin and enoxaparin, but not following dalteparin. However, no difference between groups was detectable. A small but consistent decrease of f1.2 formation in venous blood was noted for all LMWHs and dalteparin and enoxaparin, but not certoparin, inhibited TAT formation. Only a minor impact of the three LMWH preparations was noted on beta-TG plasma concentrations. Our data indicate that the studied LMWH preparations have a major impact on blood clotting in the activated state and inhibit in vivo the hemostatic system to a comparable extent.