Steroid-phosphates, from different tissues of adult female mice, were used as substrates for acid and alkaline phosphatases. The hydrocortisone-21-phosphate was split by both acid and alkaline phosphatases from tissues from the ovary, uterus, vagina, adrenal, kidney and small intestine. With this substrate, the maximum effect was obtained with alkaline phosphatase from the adrenal, ovarian and uterine tissues. The estradiol-17 ß-phosphate was split only by acid phosphatase from ovarian, uterine, vaginal, adrenal and small intestine tissues and both enzymes from kidney tissue. These results obtained were similar to those on human term placenta and show the peculiar behaviour of phosphatases with estradiol-17 ß-phosphate.