PURPOSE: To obtain a clinical diagnostic reagent effective for the early diagnosis of acute leukemia, cerebral thrombosis, malignant tumor, septicemia, etc., by separating and recovering polypeptide having specific molecular weight prepared by the plasmin digestion of human fibrinogen.

CONSTITUTION: 1 Milligram of human fibrinogen is subjected to the plasmin digestion in an aqueous medium at 25-45°C and pH6.5-8.5, in the presence of 0.01- 0.1 CU of plasmin, and after 30-60 min. of digestion, the reaction is terminated by adding an enzyme inhibitor, etc. The product is heated at 60-90°C for 60-15 min., and the resulted denatured protein is centrifuged. The supernatant liquid is fractioned by a molecular sieve, to obtain a peptide fraction A having a molecular weigtht of about 22500. The fragment A has an amino acid composition of, Asp 8.01%, Thr 11.05%, Ser 16.65%, Glu 10.18%, Pro 7.47%, Gly 16.76%, Ala 3.97%, half Cys 0.21%, Val 3.33%, Met 1.01%, Ileu 1.42%, Leu 2.78%, Tyr 1.16%, Phe 3.38%, Hys 2.33%, Lys 5.11%, Arg 5.36%; and migrates to the β-region by immuno-electrophoresis.