Computer-assisted comparison of the herpes simplex virus type 2 (HSV-2) ribonucleotide reductase large subunit (RR1) sequence with the known primary structures of other RR1 proteins revealed a motif consisting of five leucines occurring at every seventh residue between positions 409 to 437. This motif is specific to HSV RR1 proteins. A synthetic oligopeptide (LA-4) corresponding to 15 residues in the internal portion of the motif inhibited HSV-2 RR activity. In immunoprecipitation experiments, LA-4 disrupted a complex consisting of RR1, the small RR subunit and a previously uncharacterized 180K protein, apparently of cellular origin. We deduce that the LA-4 sequence represents a critical RR1 site involved in RR complex formation and enzymic activity.