A ligated mouse intestinal loop assay has been introduced to evaluate the enteropathogenicity of strains of Escherichia coli. When inoculated as whole cultures, 19 (34%) of 55 strains isolated from diarrheic infants, each of 4 strains associated with diarrhea in human adults, and 5 of 7 strains associated with porcine coli bacillosis were found capable of causing fluid accumulation in this model. In contrast, only 1 of 28 strains isolated from normal individuals was positive in this test. Sterile concentrated filtrates of positive strains cultivated in Brain Heart Infusion broth or syncase broth were also positive, whereas filtrates of negative strains did not cause fluid accumulation. All of the strains associated with diarrhea in human adults and 16 of the 19 positive strains from diarrheic children were also found capable of causing hemagglutination of chicken erythrocytes in a slide test used to evaluate the "adhesiveness" of the bacteria, a property which may enable them to stick in the small bowel and avoid the normally efficient peristaltic clearance mechanism. Few of the nonenteropathogenic strains were positive in this test. The slide hemagglutination test may be a useful, rapid means of screening potentially enteropathic E. coli strains and merits further evaluation and study. Although not ideal, the mouse intestinal loop model offers some advantages over the rabbit ileal loop assay.