The genetically engineered herpes simplex virus strains R7017 and R7020 were tested in owl monkeys (Aotus trivirgatus) previously shown to model herpetic diseases ofimmunocompromised patients and neonates. In contrast to the lethal disease seen in monkeys receiving 100–1,000 plaqueforming units (Pfu) of wild-type virus, inoculation of ⩾10⁶ pfu of recombinant viruses produced local lesions and viral shedding but not disseminated disease. Latent recombinant viruses were recovered from some ganglia innervating the sites ofinoculation. Monkeys protected from lethal infection with wild-type virus exhibit recurrent lesions that increase in frequency and severity after total lymphoid γ irradiation (TLI). In contrast, monkeys immunosuppressed by TLI and inoculated with R7020 could not be differentiated from irradiated controls with respect to morbidity or mortality. Moreover, the virus was not transmitted from immunosuppressed infected females to normal male cage mates.