A high-performance liquid chromatographic analysis is described for isoniazid as a drug entity and in its tablet and injectable dosage forms. After incorporation of the drug or dosage form in a solvent mixture and addition of an internal standard, tribenzylamine, an aliquot is chromatographed using a pellicular silica gel medium followed by UV spectrophotometric detection at 254 nm. The response of the chromatographic system was linear over a concentration range corresponding to 20-200% of the labelled amount of isoniazid. Comparison of the results with those obtained by the official USP XIX method indicates similar accuracy and precision. The advantages of the proposed method are its simplicity and rapidity, its potential for automation, and its specificity. The specificity was demonstrated in the presence of potential degradation products of isoniazid, other drugs used with isoniazid in combination dosage forms, and an adduct formed by the reaction of isoniazid with lactose in the tablet.