The synthetic nonapeptide YAGAVVNDL [identical to the nine carboxy-terminal amino acids of the small subunit of herpes simplex virus (HSV)-encoded ribonucleotide reductase (RR)] was found to inhibit the RR activity induced by equine herpesvirus type 1 subtype 1 (EHV-1). Parallel experiments with HSV type 1 (HSV-1)-encoded RR established that the concentration of peptide required to inhibit 50% of the RR activity was 28 microM for both enzymes. The optimum pH for the EHV-1 enzyme was found to be between 8.0 and 8.1 which is the same as that of HSV-1 RR. By use of antisera made against peptides corresponding to different regions of the large subunit (RR1) of the HSV-1 enzyme and monoclonal antibodies directed against HSV-1 RR1 we have obtained evidence which suggests that the EHV-1 large subunit has an Mr of approximately 90,000 and lacks the N-terminal domain which is so far unique to HSV.