The Elek test was modified for detection of the heat-labile enterotoxin of enterotoxigenic Escherichia coli. A total of 164 strains of E. coli were tested by the modified Elek test, and the results correlated well with those of the Chinese hamster ovary cell assay and passive immune hemolysis. It is concluded that the modified Elek test is a simple and reproducible assay method for identification of E. coli which produce heat-labile enterotoxin, and is suitable for use in clinical laboratories.