Students attending a Mexican university who developed diarrhea were randomly treated with bismuth subsalicylate or a placebo. One hundred and eleven were given 30 ml each 1/2 hr until eight doses (total dose of active drug 4.2 g) were given and 58 students received twice this dose (8.2 g of active drug) over the 3 1/2-hr treatment period. The number of unformed stools was significantly decreased in both bismuth subsalicylate treatment groups compared to the placebo controls for the period 4 to 24 hr after therapy. A reduction in diarrhea was additionally noted for the duration of the 48-hr surveillance period for the students receiving the higher dose of active drug. Subjective relief within 24 hr of therapy of the symptoms of diarrhea, nausea, and abdominal pain or cramps was reported with a significantly increased frequency in the bismuth subsalicylate group. The most pronounced effect of the treatment occurred in the United States students with diarrhea who had recently arrived in Mexico. This appeared to be related to the favorable effect of bismut subsalicylate on the course of toxigenic Escherichia coli infection. Students with shigellosis did not experience a prolonged illness in either treatment group.