Forty-eight strains of Escherichia coli isolated from children with diarrhea were classified according to nine enteropathogenic serotypes. The strains were examined for production of enterotoxin and for invasiveness by study of bacteria or bacteria-free filtrates in conventional animal and tissue culture models. Filtrates of only three strains (6%) consistently dilated rabbit ileal loops, while all 48 strains yielded negative results in suckling mice, adrenal cells, and guinea pig eyes. When filtrates of the three strains that dilated the rabbit ileum were heated at 60 C for 30 min, the reaction in rabbit ileal loops was negative; this finding indicated the production of a heat-labile enterotoxin. This study shows the lack of correlation between classical enteropathogenic serotypes of E. coli and presently known virulence properties in animal models. The results raise doubts about the value of serotyping E. coli isolates from sporadic cases of diarrhea. When it is suspected that an E. coli isolate is enteropathogenic, it may be important to perform more than one laboratory assay.