The absorption and utilization of thiamine propyl disulfide, an allithiamine derivative, was compared with that of thiamine hydrochloride, in normal subjects and in alcoholics with and without thiamine deficiency. Both of these congeners of thiamine enter the circulation via the portal vein from the intestinal tract; oral thiamine propyl disulfide, however, which is sparingly soluble in water, produces significantly higher blood, urine, and cerebral spinal fluid levels than thiamine hydrochloride, which is water-soluble. This is attributed to the fact that intestinal transport of thiamine propyl disulfide, unlike thiamine hydrochloride, is not rate-limited in the dose range tested. Oral thiamine propyl disulfide is effective in correcting laboratory and clinical evidence of thiamine depletion refractory to oral thiamine hydrochloride. Its administration caused a rapid increase in cerebral spinal fluid thiamine and disappearance of lateral rectus palsy in patients with Wernicke's encephalopathy. Thiamine propyl disulfide has not produced any untoward effect with long-term administration. A slight odor makes it less palatable than thiamine hydrochloride and unsuitable for addition to flour and other foods. Sugar-coated preparations of this and other allithiamine derivatives have proved useful in preventing and treating thiamine depletion syndromes in Japan and should be considered for wide use for this purpose in other areas.