Ribonucleotide reductase has been suggested as a rate-limiting enzyme in DNA synthesis, partly because activities of the enzyme in cell-free preparations are low relative to rates needed to sustain DNA replication at observed rates. Vaccinia virus, with a large duplex DNA genome, encodes both subunits of a specific ribonucleoside diphosphate reductase. In this report, we describe quantitative analysis of ribonucleotide reductase protein levels and DNA accumulation in vaccinia virus-infected cell extracts, to correlate the supply of deoxyribonucleotides with the demand for these precursors in viral DNA synthesis. To do this, we generated polyclonal antisera to TrpE fusion proteins constructed from the carboxyl termini of both subunits of viral ribonucleotide reductase. We used S1 nuclease and immunoprecipitation analysis to determine the transcriptional and translational kinetics of vaccinia virus ribonucleotide reductase expression. Enzyme activity and ribonucleotide reductase protein stability were also assayed during the time course of viral infection. Enzyme-linked immunoassays were used to quantitate protein levels, and filter hybridizations were used to measure the accumulation of viral DNA. We show that ribonucleotide reductase activity in vaccinia virus-infected cells is severalfold higher than needed to provide deoxyribonucleotides at rates commensurate with DNA synthesis. Thus, while the enzyme is important as catalyst for the first committed reaction in DNA replication, it is not rate-limiting for this process.