A heat-stable enterotoxin produced by a strain of enterotoxigenic Escherichia coli 18D was purified by ion-exchange and reversed-phase high-pressure liquid chromatography. The amino acid sequence of the purified toxin was determined by Edman-degradation and a combination of fast atom bombardment mass spectrometry and carboxypeptidase digestion to be Asn-Thr-Phe-Tyr-Cys-Cys-Glu-Leu-Cys-Cys-Asn-Pro-Ala-Cys-Ala-Gly-Cys-Tyr.