We studied the cellular enzymatic defenses against anthracycline-induced free radical damage in the HL60 human myelogenous leukemia cell line and in its anthracycline-resistant subline, HL60/AR. Intracellular glutathione (GSH) levels and gamma-glutamyl transpeptidase activity were lower in HL60/AR than in HL60 cells. Glutathione-S-transferase (GST) and glutathione peroxidase activities were similar in both cell lines. The intracellular distribution of GSH/GST was visualized by digitized video fluorescence microscopy, utilizing the fluorescent probe monochlorobimane fluorescence microscopy, utilizing the fluorescent probe monochlorobimane (MBCl), which is specifically conjugated to GSH by GST. In HL60 cells stained with the MBCl probe, a bright diffuse cytoplasmic and nuclear fluorescence pattern was observed, whereas in HL60/AR cells, the fluorescence was mostly localized to the Golgi apparatus with a lesser component of diffuse cytoplasmic and nuclear fluorescence. Pretreatment of HL60/AR cells with buthionine sulfoximine (BSO) partially reversed resistance to daunorubicin. This effect of BSO on resistance was associated not only with the abolition of localized MBCl fluorescence to the Golgi apparatus but also with increased intracellular accumulation and retention of daunorubicin. The results of our studies demonstrate that inhibition of GSH synthesis in HL60/AR cells results in significant sensitization to daunorubicin and suggest that changes in the intracellular distribution of GSH/GST and/or increased drug retention may be involved in mediating this effect.