Enzyme induction by isoniazid was studied by urinary D-glucaric acid estimation in slow and fast acetylators. Isoniazid administration increases significantively the D-glucaric acid elimination in the two classes of patients. In fast acetylators, the glucaricaciduria increases regularly up to 1.8 fold the physiological level in 30 days. In slow acetylators, after a progressive elevation during 20 days, the glucaricaciduria reaches quickly a 3 fold increase after 30 days. When a classic inducer such as phenobarbital is administrated in association with isoniazid, induction is stimulated in the two groups of patients. The estimation of plasmatic free isoniazid seems to indicate that the acetylation rate of isoniazid is not on the dependance of the induction process when the drug is administrated alone. In contrast, this rate increases when the association isoniazid-phenobarbital is administrated to slow acetylators. Possible consequences on the hepatic toxicity of isoniazid are discussed.