Abstract In preparation for studies of the time course of production in vivo of the constituent chains of rabbit fibrin, we characterized the structural features of the fibrin molecule. Fibrin was isolated from plasma and reduced and alkylated. The α, β, and γ chains were separated by CM-cellulose chromatography and their molecular weights and amino acid compositions were determined. The γ chain was sequenced 36 steps with 32 positive identifications and the β chain, 12 steps with 12 identifications. No major differences between the sequences of these chains and those of man, chicken, and dog were noted.