Irinotecan, also known as CPT-11 (I), is a potent semi-synthetic derivative of 20(S)-camptothecin (CPT). Like all known active derivatives of CPT, the lactone forms of I and its active metabolite SN-38 (II) are reversibly hydrolysed to inactive carboxylate forms. We describe a sensitive and selective HPLC method using the ion-pairing reagent tetrabutylammonium phosphate (TBAP) which allows the simultaneous determination of the carboxylate and lactone forms of I and II in human plasma samples following the precipitation of plasma proteins with an ice-cold mixture of acetonitrile and methanol. The mobile phase was 0.075 M ammonium acetate buffer (pH 6.4)-acetonitrile (78:22, v/v) containing 5 mM TBAP. Separation of the compounds was performed on a radially-compressed C18 column. The limits of quantitation in human plasma were 2 and 10 ng/ml for the two forms of II and I, respectively. In addition, we propose the use of CPT lactone both as an internal standard and as a "watchdog" for sample status. Under unsuitable storage conditions, CPT elutes increasingly in its carboxylate form thereby alerting the operator of possibly erroneous determinations of the concentrations of both forms of I and II. The retention times of the peaks of interest are well separated from the solvent front. This enables the detection of early eluting metabolites. The method was successfully used for monitoring of the two forms of I and II in patients treated with I.