Human fibrinogen (TF) has been separated into two fractions: F1 - homodimers with respect to the gamma chain, and F2 - heterodimers composed of gammaA and gamma' polypeptides. Their rouleaux-inducing properties were as follows: (1) both, at the same concentration of 0.8%, were less effective than TF; (2) F1 produced larger rouleaux even under static conditions of a hemocytometer where F2 was silent; (3) F2 induced the process when a suspension was gently sheared between microscopic slides. Since the synthetic peptide gamma'(414-427) inhibited the rouleau formation in a mixture with F2, the C-terminal amino acids of the gamma' polypeptide probably bind the molecule to the cell. The inhibition was feebly visible in the native ratio of F1/F2, implicating a compensatory effect of F1.