A polypeptide of Mr 4400 was isolated from soybean (Glycine max) seeds by extraction with 60% ethanol followed by ion-exchange and reverse-phase chromatography. The peptide contains an unusually high amount of aspartic acid residues (approximately 25%, and hence is designated as soybean aspartic acid-rich peptide). Its complete primary structure was determined by conventional methods to be the following. Ser-Lys-Trp-Gln-His-Gln-Gln-Asp-Ser-Cys-Arg-Lys-Gln-Leu-Gln-Gly-Val-Asn- Leu-Thr-Pro-Cys-Glu-Lys-His-Ile-Met-Glu-Lys-Ile-Gln-Gly-Arg-Gly-Asp-Asp- Asp-Asp-Asp-Asp-Asp-Asp-Asp A striking features of this primary structure is the presence of a poly(L-aspartic acid) sequence at the carboxyl terminus, and this polyaspartyl segment was found to precipitate bovine trypsin. The presence of the putative cell attachment sequence Arg-Gly-Asp was also noted.