Between the 24 October 1977 and the 31 December 1978 a series of 299 patients were treated with triple therapy--Rifampicin, Ethambutol and Isoniazid; the latter was given in a standard dose of 5 mg/kg. Between the 1 January 1979 and the 31 December 1981 a second series of 448 patients were treated with the same combination, Isoniazid was given in a dose adjusted for acetylator status (Vivien et coll.). In all the patients the serum concentration of Isoniazid at 3, 6 and 12 hours (after a single oral dose of 5 mg/kg) were measured and the two series of patients were comparable. No significant difference was noted between the two groups regarding either efficacy or toxicity. The adjustment of Isoniazid dosage according to acetylator status has not resulted in any statistically significant benefit in those patients studied.