Hemolysis is the fragmentation of erythrocytes into microparticles (Hb-MP). Clinical hemolysis can result in a severe procoagulant state. The influence of Hb-MP on thrombin generation was quantified. Unfrozen citrated normal plasma (five donors) was supplemented with 0 or 1 g/l Hb-MP obtained through erythrocyte destruction by hypotonic lysis, freezing/thawing, or blood oxidation with 1 or 2 mmol/l chloramine-T. Pooled normal plasma was supplemented with 0-10 g/l Hb-MP and with 0-1 IU/ml low-molecular-weight heparin (dalteparin). Samples (50 microl) were tested in the recalcified coagulation activity assay. At 10 min coagulation reaction time the hypotonic lysis of erythrocytes appears to be the most procoagulant condition, followed by twice freezing/thawing, three times freezing/thawing, and once freezing/thawing. Oxidation of whole blood with 1 or 2 mmol/l chloramine-T decreases thrombin generation by about 20 or 50%, respectively. The thrombin generation in 1 mmol/l chloramine-T or 2 mmol/l oxidized plasma decreases by about 70 or 85%, respectively. The 50% inhibitory concentrations of low-molecular-weight heparin against recalcified thrombin generation are 0.01, 0.025, or 0.035 IU/ml for plasma supplemented with 0, 0.1, or 1 g/l Hb-MP, respectively. The recalcified coagulation activity assay allows one to quantify thrombin generation in critical hemolytic samples. It is suggested to find the appropriate pharmacologic dose of low-molecular-weight heparin.