Culture supernatants from two strains of E. coli were placed into different ligated intestinal sections (loops) of each animal. The two bacterial strains were identical except that one contained a plasmid carrying the heat-stable enterotoxin b (STb) gene, while the other did not. Morphometric techniques were used to assess villous epithelial surface areas and mucosal volumes in both intestinal segments exposed to STb-positive (test) and to STb-negative (control) supernates. In pigs whose intestines were exposed to STb-positive supernatants for 2 hours, both villous epithelial surface area and mucosal volume were significantly smaller in test loops than in control loops (P < 0.02). In pig test loops incubated for 1 hour and in lamb test loops incubated for 2 hours, villous epithelial surface area decreased although not significantly (0.05 < P < 0.10). Rabbit test loops did not differ from rabbit control loops in either villous epithelial surface area or mucosal volume. Histological examination of the tissues from all three species revealed epithelial changes in pig and lamb tissues only. Intestinal tissues from pigs were examined by transmission electron microscopy, but no difference between test and control tissues was seen. We conclude that STb is capable of causing partial villous atrophy in young pigs after only 2 hours;In intact neonatal piglets, three strains of enterotoxigenic Escherichia coli (ETEC) attracted greater numbers of neutrophils into the lumen and wall of the intestine than did a nonenteropathogenic strain of E. coli. Ligated loops of small intestine in 8-week-old pigs were used in attempts to identify the attributes of ETEC involved in stimulating an increased enteroluminal migration of neutrophils. The three positive strains all produced enterotoxin STb. Neither of the negative ETEC strains produced STb. An STb-containing culture supernatant prepared from a strain of E. coli which contained an STb plasmid did not attract significantly more neutrophils than did a control supernatant prepared from the same strain of E. coli without the plasmid. It was concluded that there are increased numbers of neutrophils in the intestinal lumen during ETEC infection of newborn pigs. However, attempts to identify the attribute(s) of ETEC responsible for eliciting enteroluminal neutrophils were not successful.