Two steroid-linked phosphoramide mustard prodrugs, 7a and 7b, synthesized. The androgens testosterone and 19-nortestosterone were linked through the 17beta-position via an acetal bond to aldophosphamide (3). Proton-catalyzed, as well as cytochrome P450-mediated cleavage of the acetal bond resulted in the release of 3 which decays into the ultimate cytotoxic species, phosphoramide mustard. In a competitive cellular binding assay, the new prodrugs displayed approximately 10-12% affinity to androgen binding proteins in breast cancer cells, relative to testosterone (100%). In the sex hormone receptor-negative cell line MDA-MB231, the testosterone conjugate 7a and the 19-nortestosterone conjugate 7b have been found to be as effective as 4-hydroperoxycyclophosphamide (5). Both compounds were more active than 5 in receptor-positive cell lines. No significant differences in response were observed, however, between receptor-negative and receptor-positive cell lines.