The present study was undertaken to provide further evidence for the mechanisms proposed for the toxicity of ingested bean lectins in animals: to show the stability of concanavalin A (Con A) in the gastrointestinal tract so that it has enough time to interact with some enzymes localized in the intestinal membrane and to find its effect on the activities of those enzymes that have been adopted as criteria for adaptive changes in response to altered diets, namely intestinal sucrase, alkaline phosphatase and leucine aminopeptidase. Significant amounts of ingested Con A were recovered unaltered (as seen from affinity chromatography and electrophoresis) from the cecal content of rats 4 h after its oral administration and from feces (90% recovery) 4 d later. This indicated that Con A is quite stable during its passage through the gastrointestinal tract. Con A, given at a level of 0.3 or 0.5% in the diet, completely prevented adaptive changes in the activities of those enzymes. These results substantiate the mechanisms proposed earlier by other investigators that the toxicity of ingested bean lectins involves their binding to the luminal surface of the small intestine, where they disturb the function of the brush border membrane.