The pathogenicity of a strain of simian herpesvirus SA8 in one month old conventional and gnotobiotic baboons was investigated. Intratracheal inoculation resulted in a mortality rate of 1/5 in the conventional and 1/4 in the gnotobiotic group. Disease became apparent after 3 days and was characterized by respiratory distress, reduced formula intake, weight loss and fever in both groups. Isolation of herpesvirus from the respiratory tract, lymphoid organs, kidneys, adrenals, and CNS was more frequent by explant culturing than by routine procedures. Although there was a significant difference in total white blood counts (WBC), with higher values in conventional vs. gnotobiotic infants, the absolute number of lymphocytes was not different. The lower number of WBCs apparently was due to fewer polymorphonuclear leukocytes in the gnotobiotic baboons. Infection resulted in a leukopenia 5 days post infection (p.i.) and a leukocytosis 10 days p.i. in both groups. The animals, which succumbed, had acute necrotizing fibrinous pneumonia. Intranuclear inclusion bodies typical for herpesviruses were present. All the surviving infant baboons had subacute interstitial pneumonia, when sacrificed 35 days p.i.