Survival and changes in mean body mass of whole-body irradiated mice were determined to examine the radioprotectant activity of 5-diethylsulfonamoylsalicylatocopper(II) [Cu(II) (5-DESS)]. One of four groups of 25 female C57BL/6 mice were treated subcutaneously (sc)with 0, 10, 20, 40, 60, 80, 100, or 120 mumol Cu(II)(5- DESS)/kg of body mass 3 hours before exposure to 8.0 Gy, gamma irradiation. In this paradigm, doses of Cu(II)(5- DESS) increased survival up to 92% above vehicle-treated control mice (P = 0.008). Mean body mass determinations revealed that mice treated with 80 to 120 mumol Cu(II)(5-DESS)/kg of body mass exhibited a smaller decrease in body mass than other complex-treated groups. These results support the hypothesis that Cu(II)(5-DESS) is an effective radioprotectant.