Rhodium II citrate was tested in mice for acute toxicity, antitumoral activity against Ehrlich ascites carcinoma and inhibition of DNA synthesis by Ehrlich tumor, malignant adrenocortical cells (Y-1) and normal adrenocortical cells (AR-1). At ip doses up to 260 mg/kg, the compound had no toxic effects for up to 14 days. The same total dose given over 4 days significantly increased the survival rate of mice bearing Ehrlich ascites cells. Thymidine incorporation by Ehrlich tumor, Y-1 and AR-1 cells in vitro was inhibited 50% by 0.1 to 0.2 mM concentrations of the compound. We conclude that the increased survival of the tumor-bearing mice was due at least in part to the inhibition of DNA synthesis with a consequent reduction of cell division and tumor growth.