The responsiveness of enterocytes to Escherichia coli heat-labile enterotoxin (LT) was studied in the small intestine of 6- to 7-week-old rats. Dose-effect analysis showed the dose required for a 50% maximal LT-induced secretory response to be at 8 nM. After the well-documented glycolipid GM1 receptor was blocked with the cholera toxin B subunit, LT still activated the second messenger cascade, measured in terms of heightened cellular adenylate cyclase activity, and caused fluid to be secreted into ligated intestinal loops. Furthermore, Scatchard analysis of binding kinetics suggested that LT bound to two receptor sites on the intestinal microvillus membrane. The toxin also bound to delipidated membrane but was competitively inhibited by a galactose-specific lectin, RCA60, suggesting that the additional receptor is a galactoglycoprotein. Western blot analysis of toxin binding to membrane proteins revealed a group of binding components around 85 to 150 kilodaltons. When measured at 2.2 nM LT, approximately 70% of LT-binding activity took place through a high-affinity (Kd1, 0.38 nM) GM1 receptor and 30% of LT-binding activity took place through a low-affinity (Kd2, 3.3 nM) glycoprotein receptor. These results suggest that LT functions through two microvillus membrane receptors in the mature rat small intestine.