A high performance liquid chromatographic (HPLC) assay is described for the determination of the investigational anticancer drug 9 aminocamptothecin (9-AC) as the lactone form (9AC(lac)) and as the total of the lactone and hydroxycarboxylate forms (9AC-(tot)), in micro volumes of plasma. The analytical methodology reported here involves a protein precipitation step with cold methanol (-30 degrees C) as sample pretreatment procedure. The methanolic extract is used for the determination of 9AC-(tot). The intact (active) lactone form of 9-AC is separated from the hydroxycarboxylate form in the methanolic plasma extract by solid phase extraction within 48 h after sampling and deproteination. After evaporation to dryness (nitrogen, 40 degrees C) the extract can be stored at -70 degrees C for at least 3 weeks. The drug is analysed by reversed-phase liquid chromatography on a Zorbax SB RP-18 column, using methanol-water eluent (pH 2.2) and fluorescence detection. The presented assay is linear over a concentration range 0.2-100 ng.ml-1 with a detection limit and a limit of quantitation of 0.05 and 0.2 ng.ml-1, respectively, for both 9-AC(tot) and 9-AC(lac) using a 100 ml plasma sample. The proposed method has been implemented in a phase I clinical trial for pharmacokinetic evaluation of this potential new drug.