In two consecutive steps, thrombin cleaves the fibrinopeptides A and B from fibrinogen producing des-A fibrin and des-AB fibrin. Labeled des-A fibrin was prepared by batroxobin and labeled des-AB fibrin by clotting of 125I-fibrinogen with thrombin. Fibrin solubilized in buffered urea was mixed with plasma containing 131I-fibrinogen (fibrin:fibrinogen ratio = 1:20). These fibrinfibrinogen mixtures were applied to sepharose CL- 6B columns eq ui librated with buffered plasma (0.0025 M EDTA, 0.1 M NaCl, 0.05 M tris, 0.005 M EACA, 2 AT U hirudin/ml, 500 KIU a protinin/ml, 0.003 M NaN3, pH 7.4). Plasma was used as an equilibration and elution medium to prevent precipitation of fibrin in the columns. At 20°C, labeled des-A fibrin as well as des-AB fibrin were eluted in the void volume as high-molecular weight aggregates (peak A) and separated from m onomeric labeled fibrinogen (peak B). At 37°C, however, des- A fibrin was eluted at the same position as monomeric fibrinogen (peak B), whereas des-AB fibrin was eluted in the void volume as at 20°C. Rechromatography of isolated fractions of peak A and peak B at different temperatures showed that monomeric fibrin isolated at 37°C formed high molecular weight material at 20°C, and high-molecular weight fibrin isolated at 20°C dissociated at 37 ° C. The results suggest that des-A fibrin solubilized in plasma in the absence of calcium ismonomeric at 37°C but forms high-molecular weight aggregates at lower temperature, whereas des-AB fibrin is oligomeric at 20°C as well as at 37°C.