We found and identified four heterozygous dysfibrinogenemias with AalphaR16H(CGT-->CAT) mutation in two families by coagulation tests and direct sequence analysis for PCR-amplified DNA fragments. Two dysfibrinogens were designated as fibrinogen Toyama and Adachi, according to the place of residence of proposituses, respectively. Patients' fibrinogen purified from plasma using immunoaffinity-chromatography was subjected to thrombin- or batroxobin-catalyzed fibrin polymerization, fibrinopeptide A (FPA) release, and clottability test. AalphaR16H-fibrinogen showed impaired thrombin or batroxobin-catalyzed fibrin polymerization in comparison with normal control fibrinogen. It is interesting that the period of protofibril formation of Toyama propositus was longest in those of four affected people. The clottability of AalphaR16H-fibrinogen was 66-70% with thrombin and higher than with batroxobin, 35-50%. In the same condition with fibrin polymerization, thrombin and batroxobin did not cleave the Aalpha16H-17G peptide-bonding, resulting in no release of variant FPA. From these results, we speculated that elongation of the two-stranded protofibril formation would be terminated by participation of the heterodimer fibrinogen molecules composed with a normal and an aberrant Aalpha-chain, and it would result in a decrease in fibrin polymerization. We speculated that the difference in the extent of impairment of fibrin polymerization among the patients might be caused by the different amount of heterodimers. Moreover, we also speculated that batroxobin-induced clottability was lower than thrombin-induced clottability, because batroxobin cannot induce the so-called "B-knob-b-hole" interaction, which enhances fibrin formation.