The large subunit of ribonucleotide reductase from mouse has been overexpressed in Spodoptera frugiperda cells infected with recombinant baculovirus. The expressed protein was purified by affinity chromatography to apparent homogeneity as determined by SDS-PAGE. The homogeneous protein is recognized in Western blot analysis by a monoclonal antibody raised to the large subunit of ribonucleotide reductase from calf thymus, has the correct N-terminal sequence, and, in the presence of the small subunit of mouse ribonucleotide reductase and nucleoside triphosphate effectors, catalyzes the reduction of both purine and pyrimidine nucleoside diphosphates.