The impact of delayed fibrinopeptide-A release on polymerization and structure of fibrin gels was studied utilizing a heterozygously transmitted variant fibrinogen. An arginine to histidine substitution at position 16 of the alpha chain of the abnormal fibrinogen delayed release of an abnormal fibrinopeptide-A (A) by thrombin and completely blocked release of A by reptilase. When clotted with thrombin, patient fibrin formed more slowly than normal fibrin, but clottability was normal and gel fiber mass/length ratios were decreased less than 10%. Gels formed with reptilase clotted slowly, demonstrated reduced clottability, but had normal fiber mass/length ratios. Reptilase clotted the normal but not the variant component of the patient fibrinogen. Thrombin-induced cleavage of fibrinopeptide-B prior to A occurred in these experiments, but polymerization of this species beyond trimers has been reported to be minimal under the conditions used. With time, A is removed by thrombin resulting in the slow production of normal fibrin monomer from the abnormal component. These monomers subsequently polymerize. The minimal change in gel fiber size caused by slow A release implies that fibrin fiber size is primarily a function of ionic environment and not of the sequence of peptide release.