When dosing isoniazid in patients' serum after ingestion, the authors find a difference in the isoniazid levels when isoniazid is used alone, then, when isoniazid is used simultaneously with rifampicin. The level decreases or increases according to the inactivation index. They analyse the therapeutic consequences which can result from it. They infer, from the practical point of view, the necessity of : dosing isoniazid after having ingested it simultaneously with rifampicin and not alone when the efficient therapeutic dose is to be determined ; testing serum levels during antituberculous therapy for an eventual readjustment.