The biologic activity in terms of survival of normal hematopoietic stem, osteosarcoma, and L1210 leukemia cells was determined for the following compounds:cyclophosphamide, its derivatives isophosphamide and trophosphamide, its possible metabolites nor-nitrogen mustard, hydroxylamine mustard, 4-ketocyclophosphamide, and and acrolein, and two substitutes for its primary active metabolite 4-hydroxycyclosphamide anhydro-dimer (4-hydroxy-CP-anhydro-dimer) and 4-hydroperoxycyclophosphamide (4-hydroperoxy-CP). On a molar basis none of the compounds shows a better therapeutic ratio between osteosarcoma and bone marrow stem cells than the parent compound. The therapeutic ratio between L1210 leukemia and normal cells is slightly better for 4-hydroperoxy-CP only. It may be concluded that the conversion of 4-hydroxy-CP-anhydro-dimer and 4-hydroperoxy-CP to the primary active metabolite 4-hydroxycyclophosphamide differs quantitatively. Moreover, it appears that by the use of these precursors a better therapeutic ratio might be obtained for some malignancies but not for others.