1. The intestinal absorption of [35S]thiamine hydrochloride was investigated in healthy subjects, malnourished alcoholics and a patient with resection of the jejunum and ileum. Serum and urinary radioactivity was studied after administration of 10 μCi of [35S]thiamine hydrochloride in 1–50 mg of non-radioactive thiamine hydrochloride. 2. Results suggest that intestinal absorption of thiamine hydrochloride is rate-limited. Though the results provide only indirect information on intestinal transport rates, they are consistent with the Michaelis-Menten relationship used to describe enzyme-substrate reactions. Calculations by this model yielded a Vmax. of 8·3 ± 2·4 mg and a Km of 12·0 ± 2·4 mg for normal subjects with a significant decrease in Vmax. in malnourished alcoholics and a patient with resected small intestine. 3. Intestinal absorption and the calculated value of Vmax. for thiamine hydrochloride is increased in malnourished alcoholics after correction of malnutrition. These findings are consistent with the thesis that this vitamin is absorbed by a saturable mechanism and that the number of effective receptor sites may be reduced by malnutrition or intestinal resection.