Chicken fibrinopeptide A was isolated and the complete amino acid sequence was established. It is a pentadecapeptide with the sequence of Gln(Pyr)-Asp-Gly-Lys-Thr-Thr-Phe-Glu-Lys-Glu-Gly-Gly-Gly-Gly-Arg and is homologous with mammalian fibrinopeptide A. Two peptides which appear to be derivatives of fibrinopeptide A were also isolated. One of these appeared to be fibrinopeptide A with NH2-terminal pyroglutamic acid; the other was fibrinopeptide A which lacked the COOH-terminal arginine residue.