We describe a sensitive and expedient microassay of isoniazid in plasma by use of electrochemical detection in conjunction with a radial compression liquid chromatography. Diphenylcarbazide was used as an internal standard and the separation was achieved on a 10-micron, 8 mm x 10 cm C18 (12% carbon load) cartridge. A mixture of 10 mM sodium dibasic phosphate solution (adjusted to pH 7 with phosphoric acid) and methanol (93.5:6.5, by volume) was used as a mobile phase at a flow rate of 4 ml/min. After dilution with an equal volume of aqueous solution of the internal standard (6.667 microM), the plasma sample (50-150 microliters) was deproteinized by use of an Amicon Centrifree-MS ultrafilter at 2,000 g. No interference from any endogenous substance or concomitantly used drug was observed, and the assay was highly linear (r greater than 0.9994) in the concentration range examined (0.146-145.8 microM). The intra- and interrun precision was equally good, with coefficients of variation of less than 3.13 and less than 5.4%. We are currently using this method for monitoring isoniazid in patients treated with this drug for tuberculosis.