An analytical method was developed for the anticancer agent irinotecan (CPT-11) and its main metabolite SN-38 in human whole blood and in red blood cells (RBCs). Sample pretreatment involved deproteinization of whole blood or plasma-diluted RBCs isolated by MESED instruments, with a mixture of aqueous perchloric acid and methanol (1:1, v/v). Separation was carried out using isocratic elution on a Hypersil ODS stationary phase, with detection at excitation and emission wavelengths of 355 and 515 nm, respectively. The lower limit of quantitation (LLQ) in blood was established at 5.00 ng/ml for both compounds, with values for within-run precision (WRP) and between-run precision (BRP) of less than 10%. The method is currently being applied to investigate the blood distribution of CPT-11 and SN-38 in cancer patients.