Assessed by high performance liquid chromatographic and amino acid sequence determinations, approximately one half (n = 4) of A peptide in fibrinogen Stony Brook (phi SB) contained the A alpha 16Arg----Cys substitution. To examine its functional behavior, mutant molecule-rich soluble subfractions that partly or fully lacked their normal A peptide were obtained from cryoprecipitates or from incoagulable material, respectively. Such subfractions consistently induced a more pronounced decrease (n = 3) in the turbidity of normal polymerizing fibrin than that induced by normal fibrinogen, by whole phi SB (n = 4) or by fibrinogen from an unrelated homozygous proband. These subfractions also exhibited decreased (12-50% of normal controls, fibrinogen 30-590 nM, n = 5) ADP-induced aggregation support of gel-sieved platelets, a decrease not demonstrable by whole phi SB, by fibrinogen from the homozygous proband, or by enrichment of the latter with normal soluble fibrin. A single isolate displaying diminished platelet aggregation support was 125I-labeled and examined further. It exhibited decreased binding to platelets, and Scatchard analysis indicated decreased binding affinity but normal maximum binding. We infer that phi SB contained heterodimers that exhibited these distinct functional properties when their normal A peptide had been cleaved.