The spin labeled analog of Thio-Tepa, 1-oxyl-2,2,6,6-tetramethyl-4-piperidyl-N,N;N′,N′−bis (ethylene)−phosphorodiamidothioate (SL−O−TT), which contains a nitroxyl free radical linked by an oxygen bridge to phosphorus, has antitumor properties against P388 murine leukemia (T/C = 242) and a higher therapeutic ratio (5.15) than its parent compound, Thio-TEPA (2.75). The drug is less toxic to P388 cells in culture as judged by the 3 H-thymidine uptake. On the basis of electron spin resonance spectroscopy using L1210 cells incubated with SL−O−TT, it is concluded that the drug is bound to cells in culture in such a way as to restrict the motion of the nitroxyl label. A second spin labeled analog, 1-oxyl-2,2,6,6-tetramethyl-4-amino-piperidyl-N,N;N′,N′−bis (ethylene)−phosphorodiamidothioate (SL−NH−TT), containing a nitroxyl label linked by a nitrogen bridge to phosphorus, first synthesized by Russian workers, was prepared by an improved procedure in 95% yield. In vivo results indicate that this analog has about the same therapeutic value (2.73) as Thio-TEPA (2.75), and that higher doses of this compound are required than those for both the O-bridged analog and Thio-TEPA to achieve maximum T/C values.