In view of the in vivo demonstrated radiation-potentiating activities of several previously studied 2,2-dimethylphosphoraziridines, six new compounds incorporating the bis(2,2-dimethyl-1-aziridinyl)phosphinyl moiety, together with an electron-affinic group such as 2-nitroimidazole or nitrobenzyl, have been synthesized and tested (1) in vitro for ability to increase the effect of X-irradiation under hypoxic conditions on V-79 Chinese hamster lung fibroblast cells, (2) in vivo for antitumor activity in the absence of radiation against P388 leukemia in mice, and (3) in a preliminary experiment with compound 10 only, in combination with whole-body gamma-radiation, using the P388 leukemia mouse model for in vivo radiation-potentiating activity. The chemical-alkylating activities and hydrolytic behavior of these compounds, as well as their antitumor activities without radiation, were found to be comparable to those of other 2,2-dimethylphosphoraziridines, while their in vitro radiosensitizing activities were at low concentrations generally comparable to that of misonidazole, with compound 8 showing superior activity. At higher concentrations, only compound 10 was sufficiently soluble and nontoxic to the cells for evaluation in this assay. Thus, the bis(2,2-dimethyl-1-aziridinyl) phosphinyl moiety does not seem to have contributed to the hypoxic radiosensitizing activities (only to the cytotoxicities) of the electron-affinic moieties in this in vitro assay. In comparison, the prototype 2,2-dimethylphosphoraziridine, ethyl [bis(2,2-dimethyl-1-aziridinyl) phosphinyl]carbamate (AB-132), showed at nontoxic doses no radiosensitizing activity in this assay, and at cytotoxic doses increased the cell-killing effect of each given dose of X-radiation additively under both hypoxic and oxic conditions. Conversely, only the 2,2-dimethylphosphoraziridine moiety appeared to participate in the moderate "therapeutic radiation-potentiating" activity indicated by compound 10 in the in vivo experiment using the P388 leukemia model (on day 1), as the misonidazole standard was inactive in this nonhypoxic system. Clearly, the mechanism of the in vivo observed radiation-potentiating effect of AB-132 and other 2,2-dimethylphosphoraziridines is different from that of the hypoxic radiosensitizers, but the possible synergism between the two biologically active moieties of the new compounds could not be demonstrated with the experimental models so far employed.