We describe preliminary studies of a new familial dysfibrinogen, fibrinogen Stony Brook II, present in a propositus and his mother. Both presented with a history of unexplained and chronic joint swelling following trauma, and the propositus suffered recurrent knee haemarthroses following arthroscopic surgery. Prolonged plasma thrombin times required a 3-5 fold excess of normal plasma for correction. Isolated fibrinogen displayed prolonged clotting times and delayed onset of clot turbidity. Also, an abnormal peptide could be released by thrombin but not by batroxobin along with approximately half the expected amounts of normal A peptide. Assessed by its thrombin release and by its early HPLC retention position, the abnormal peptide suggests a possible A alpha-16-Arg----His substitution. The data suggests an association in these probands between this heterozygously transmitted anomaly and the apparently impaired healing in hypovascular sites.