The clinically isolated heat labile enterotoxin (LT)-producing strains of Escherichia coli can be separated into two groups, namely spontaneous LT-releasing strain and non-spontaneous LT-releasing strain, based on their phenotypes of spontaneous release of LT into the culture medium. The phenotype of spontaneous LT release was observed to correlate closely with the phenotype of the release of numerous small vesicles into the culture medium. Both morphological and biological examinations of the vesicle showed that the vesicle was released from the outer membrane. It can, therefore, be assumed that LT may be released from the cell at the time the vesicles form.