A previous study had established a relationship between the major polypeptide of the Epstein-Barr Virus (EBV)-induced restricted (R) component of the early antigen (EA) complex and the large subunit of ribonucleotide reductase. This association was confirmed in this study by the observation that a monoclonal antibody prepared against the 85-kDa R component reacted in immunoblotting with the protein product (molecular weight approximately 93 kDa) encoded by the Xbal fragment of the B-95-8 strain of EBV. This fragment encodes for both the small and large subunits of this virus-induced enzyme. This size of the reactive protein indicated that it was the large subunit. Direct evidence that the 85-kDa EA-R component was associated with the viral-induced ribonucleotide reductase was provided by enzyme inhibition studies. Two monoclonal antibodies prepared against the tertiary structure of the 85-kDa EA-R antigen significantly inhibited the in vitro activity of the viral-specified enzyme but had no effect on ribonucleotide reductase activity derived from two different cellular preparations. A monoclonal antibody prepared against the denatured form of this antigen was ineffective in neutralizing the virus-specific ribonucleotide reductase as were antibodies to different EBV polypeptides. These results therefore conclusively demonstrate that the 85-kDa EA-R polypeptide is associated with the large subunit of this viral-specified enzyme and further suggest that the active site of the enzyme is associated with the tertiary structure of the large subunit.