AbstractHuman breast cancer cell lines have been shown to possess high affinity receptors for 1,25‐dihydroxyvitamin D3 (1,25(OH)2D3) on the activity of glucose‐6‐phosphate dehydrogenase (G6PD) in cells of a human breast cancer cell line MCF‐7. G6PD, an enzyme which controls the hexose monophosphate shunt, is elevated and sensitive to 17β‐estradiol in breast tumors. G6PD activity was stimulated by 1,25(OH)2D3 in a dose‐dependent manner at very low concentrations of steroid (10−10–10−12M). 1,25(OH)2D3 increased maximum velocity without modifying the affinity constant of the enzyme for g