1. Daunorubicin reductase, an enzyme ubiquitous in all mammalian tissues tested, converts daunorubicin, a cancer chemotherapeutic antibiotic, to daunorubicinol. A method for the estimation of daunorubicinol and daunorubicin reductase is described.2. Daunorubicin and daunorubicinol were quantitatively extracted from enzyme assay mixtures with 0.3 N HCl in 50% ethanol and then completely hydrolysed to their respective aglycones. The aglycones were extracted, concentrated into toluene, separated by silicic acid microcolumn chromatography, and estimated by spectrofluorimetry.3. Compared to previous methods, this method is highly reproducible and sensitive.