Herpes simplex virus type 1 encodes a ribonucleotide reductase (RR) consisting of two subunits (140 and 38 kDa) whose genes map to coordinates 0.56 to 0.60 on the viral genome. We previously reported the isolation and characterization of a mutant with a lacZ insertion into the large subunit (ICP6) gene (Goldstein and Weller, 1988). Studies with this blue-plaque mutant, hrR3, showed that the viral RR activity is not essential in dividing cells in culture. This mutant, however, synthesizes the N-terminal one-third (434 amino acids) of ICP6 which may have an additional, required function. To test this possibility, a deletion of the ICP6 gene was created by introducing a deleted ICP6 gene into infectious hrR3 DNA and screening for white plaques from a background of blue plaques. Studies with this mutant, ICP6 delta, demonstrated that ICP6 is not required for virus growth and DNA synthesis in dividing cells in culture. However, we show that the ability of ICP6 delta to grow and induce viral DNA synthesis is dependent on the state of the infected cells; ICP6 delta is severely compromised in nondividing cells or in cells at 39.5 degrees. We propose that an alternate pathway(s) for obtaining deoxyribonucleotides is operating in infected cells and can compensate for defects in viral RR. In addition, our experiments suggest that these alternate sources are not available either in nondividing cells or in cells at 39.5 degrees.