There is increasing interest concerning the use of natural antioxidants as low toxic antileukemic compounds. Antoksyd S (C/E/XXI), is a novel herbal drug derived in Poland from the powdered roots of Scutellaria baicalensis, and the biological activities of its major components (baicalin and baicalein) were compared on the human leukemia cell line HL-60. On MTT assay, Antoksyd S (C/E/XXI) showed an obvious cytotoxic effect on HL-60 cells, which was compared with those caused by cisplatin and doxorubicin under the same experimental conditions. A comparative assay of the antioxidative and prooxidative capacity of Antoksyd S (C/E/XXI) was also undertaken using two different reactive species: superoxide (O2-) and a transition metal (Cu2+). Antoksyd S (C/E/XXI) has low toxicity, acting as a modifier of HL-60 cells proliferation and as an antioxidant, which could act prooxidatively in the presence of transition metal ions. Taken together, it seems reasonable to suggest that Antoksyd S (C/E/XXI) as compared to baicalin and baicalein, or to the cytostatics cisplatin and doxorubicin, might be an especially good candidate for the future development of new therapeutic techniques, alone or in "combination treatment regimens", to enhance leukemia cell killing.