Available evidence which may aid a decision concerning which of the thioamides, ethionamide or prothionamide should be recommended for use in the treatment of lepromatous leprosy is inconclusive. The drugs possess similar antimycobacterial activities, but earlier work has suggested that after oral dosage ethionamide may give rise to higher blood levels than prothionamide. We report on investigations designed to examine whether this finding is as a result of different systemic availabilities, by comparing blood levels fo llowing intravenous and oral administrations. We conclude that the drugs' pharmacokinetics are very similar, each having high bioavailabilities, and that other fa ctors such as cost may be more important determinants as to which thioamide should be used.