Ribonucleotide reductase from mammalian cells consists of two nonidentical components which are both required for enzymatic activity. It was found that the addition of the effector-binding component (dye fraction) to cell-free extracts of Ehrlich tumor cells stimulated CDP reductase activity. The decrease in CDP reductase activity which accompanied the decrease in Ehrlich tumor cell proliferation in vivo could be correlated with the decrease in the dye fraction component. In regenerating liver, the reductase activity was increased maximally at 36 h following partial hepatectomy. This activity could be further stimulated by exogenous tumor cell dye fraction. The non-heme iron component (Tris fraction) was isolated and quantitated from the liver extracts of regenerating livers. The maximal increase on the Tris fraction component was observed in the 24-h regenerating liver. These data provide evidence that the components making up the active ribonucleotide reductase species are not coordinately increased at the time of the increase in reductase activity.