A reversed phase high performance liquid chromatographic procedure was developed to quantify 6-thioguanine, 6-mercaptopurine, methylthioguanine, and methylmercaptopurine in red blood cells. The free base of each thiopurine was liberated from the respective nucleoside and nucleotide moiety by acid hydrolysis, which allowed for a determination of the total thiopurine present. 6-Thioguanine and 6-mercaptopurine were analyzed on an octadecyl- silane column using methanol + 20 mM sodium phosphate (15:85), pH 7.5, containing 0.07% tetrabutylammonium chloride. Detection was by potassium permanganate oxidation and fluorescence detection at 290 nm excitation and 400 nm emission. Methylmercaptopurine and methylthioguanine were analyzed on a cyanopropylsilane column using methanol + 40 mM sodium phosphate (18:82), pH 2.7, and then ultraviolet absorption at 314 nm and 290 nm, respectively. The method was used to quantify the four primary thiopurines present in red blood cells of an acute lymphoblastic leukemia patient. The procedure may be a therapeutic monitoring technique that quantifies the cytotoxic drug burden in patients receiving azathioprine or 6-mercaptopurine therapy.