A comparison of the cytidine 5'-diphosphate (CDP) and adenosine 5'-diphosphate (ADP) reductase activities from Ehrlich tumor cells was made to determine if the properties of the enzyme for these substrates were the same, except for the allosteric effector. It was observed that various purification steps did not result in an enzyme fraction that had a constant ratio of CDP:ADP reductase activities. The optimal Mg2+ ion concentration for CDP reduction was 3 to 4 mM, while the optimal Mg2+ ion concentration for ADP reduction was 0.1 mM inhibited ADP reduction. CDP reduction was relatively insensitive to the presence of dimethylformamide or dimethyl sulfoxide in the reaction mixture, but ADP reduction was decreased in the presence of these two compounds. Periodate-oxidized adenosine 5'-monophosphate, on incubation with the enzyme, had a greater effect on CDP reduction but little or no effect on ADP reduction. The response of the CDP and ADP reductase activities to the same negative effector was essentially the same. Both CDP and ADP reductions showed similar decreases in the presence of various concentrations of deoxyadenosine 5'-triphosphate. These data suggest that the Ehrlich tumor cell reductase enzyme system could consist of at least two different enzymes that may be regulated by the same allosteric protein.