Serum and tissue distribution of doxorubicin (DX) and its stereoisomer, 4''-epidoxorubicin (4''-epiDX), after i.v. injection of 15 mg/kg was investigated in C57BL mice bearing 14-day i.m. [mouse] Lewis lung carcinoma cells. Total fluorescence was measured together with unchanged drugs, separated and quantitated by means of TLC combined with a scanning fluorescence technique. In serum the disappearance of 4''-epiDX paralleled that of DX; both unchanged isomers represented < 50% of the total fluorescence measured as early as 30 min after drug injection. In tissues the fluorescence measured was almost completely accounted for by the native compounds, DX and 4''-epiDX, indicating that these drugs are taken up as such by tissues. 4''-EpiDX levels were markedly lower than those of DX in tumor and spleen; in heart, liver and kidneys the concentrations of the 2 isomers were the same. Traces of doxorubicinol and possible 4''-epidoxorubicinol were detected only in serum, liver and kidneys. Comparative cumulative biliary excretion of DX and 4''-epiDX investigated in the rat indicated that a total of 40%-45% of the injected dose of both drugs was excreted as unchanged compound or as reduced metabolite. The proportions were different; the presence of twice as much reduced metabolite and smaller amounts of native 4''-epiDX suggests that it metabolic rate is different from that of DX.