Fibrinogen was purified from fresh citrated human plasma by precipitation with beta-alanine in the presence of citrate and protease inhibitors. From this material, fractions corresponding to the HMW (high molecular weight), LMW and LMW' fibrinogen fractions of plasma were obtained by step-wise precipitation with ammonium sulfate. Electrophoresis revealed that HMW was contaminated with 4% LMW, LMW was contaminated with 6% HMW, and the LMW'-fraction was a mixture of LMW' (50%), LMW (20%) and two derivatives of intermediate m.w.. The HMW fraction (mw. 340 000) contained intact Aa-chains, while the molecular weight of LMW was reduced to 305 000 and that of LMW' to 270 000 due to proteolysis of the -COOH terminal end of one (LMW) or both (LMW') Aa-chains. The clottability of HMW was 98%, of LMW 92% and of LMW' about 80%. Thrombin clotting times (1 NIH U/ml) were 14", 20" and 25" resp. These differences were highly accentuated when clotting was performed with reptilase (14", 38" and 120"). Contamination with soluble fibrin was less than 2% and the contents of fibronectin and AT-III were low. No thrombin or plasmin activity was generated upon 24 hours incubation at +4 degrees C as evidenced by increased content of fibrinopeptide-A and fragment Bb-15-42 resp.