The primary structure of cholera toxin B subunit, responsible for the binding of the toxin to cell surface ganglioside Gm1, has been determined as: Thr-Pro-Gln-Asn-Ile-Thr-Asp-Leu-Cys-Ala-Glu-Tyr-His-Asn-Thr-Gln-Ile-His-Thr-Leu-Asn-Asn-Lys-Ile-Phe-Ser-Tyr-Thr-Glu-Ser-Leu-Ala-Gly-Lys-Arg-Glu-Met-Ala-Ile-Ile-Thr-Phe-Lys-Asn-Gly-Ala-Thr-Phe-Glu-Val-Glu-Val-Pro-Gly-Ser-Gln-His-Ile-Asp-Ser-Gln-Lys-Lys-Ala-Ile-Glu-Arg-Met-Lys-Asn-Thr-Leu-Arg-Ile-Ala-Tyr-Leu-Thr-Glu-Ala-Lys-Val-Glu-Lys-Leu-Cys-Val-Trp-Asn-Asn-Lys-Thr-Pro-His-Ala-Ile-Ala-Ala-Ile-Ser-Met-Ala-Asn The number and the sequence of amino acid residues has been established by separation and analyses of the peptides obtained through enzymatic and chemical cleavage of the B subunit. The chymotryptic peptides from the citraconylated B subunit proved to be useful in the alignment of the tryptic peptide.