A total of 1,335 Escherichia coli strains isolated from patients with diarrhea were submitted to the Serény test. Of these strains, 1,035 were also submitted to slide agglutination test, using antisera for invasive E. coli serogroups. Of 325 isolates that were lysine decarboxylase negative, 138 reacted with one of the E. coli antisera used. From this group of 138, 137 were positive in the Serény test. Strains that were lysine decarboxylase positive (1,010 strains) were all negative in the Serény test. These results show a close correlation between the biochemical and serological characteristics of E. coli and the results of the Serény test.