Total body neutron activation analysis was employed to measure total calcium in C3H/St (Ha) female mice. Ninety-nine percent of total body calcium is in bone and loss of calcium was used as an index of osteoporosis. Heparin (500 U/kg B.I.D.) treatment for three months resulted in significant osteoporosis. 17 beta-Hydroxy-4-estren-3-one 17-decanoate 1.5 mg/kg or 4 mg/kg twice monthly prevented this heparin accelerated osteoporosis. The results suggest that 17 beta-hydroxy-4-estren-3-one 17-decanoate may be capable of preventing bone loss and partially increasing bone mass in patients exposed to osteoporosis inducing regimens.