Recently, a homologue of the small subunit of mammalian ribonucleotide reductase (RNR) was discovered, called p53R2. Unlike the well characterized S phase-specific RNR R2 protein, the new form was induced in response to DNA damage by the p53 protein. Since the R2 protein is specifically degraded in late mitosis and absent in Go/G1 cells, the induction of the p53R2 protein may explain how resting cells can obtain deoxyribonucleotides for DNA repair. However, no direct demonstration of RNR activity of the p53R2 protein was presented and furthermore, no corresponding RNR large subunit was identified. In this paper we show that recombinant, highly purified human and mouse p53R2 proteins contain an iron-tyrosyl free radical center and both proteins form an active RNR complex with the human and mouse R1 proteins. UV irradiation of serum starved, Go/G1 enriched mouse fibroblasts, stably transformed with an R1 promoter-luciferase reporter gene construct, caused a threefold increase in luciferase activity 24 hours after the irradiation paralleled by an increase in the levels of R1 protein. Taken together, our data indicate that the R1 protein can function as the normal partner of the p53R2 protein and that an R1/p53R2 complex can supply resting cells with deoxyribonucleotides for DNA repair.