Cell-mediated immune responses in 27 infants and children with cytomegalovirus (CMV) infetion acquired between birth and 1 year of age were compared with responses in 13 children who had neonatal herpes simplex virus (HSV) infection. Infection was asymptomatic in 25 of 27 CMV-infected children; the 13 patients with HSV infection were all ill as newborns. The median age when studied was 46 months for children infected with CMV and 24 months for those infected with HSV. We measured lymphocyte transformation responses (LTRs) to CMV antigens in the former group and to HSV type 1 (HSV-1) (and in six cases to HSV-2) in the latter group with the results expressed as a stimulation index. Based on the results in seropositive and seronegative adult control subjects, stimulation indexes of ≥3 were considered indicative of a positive LTR. Among the CMV-infected children, a positive LTR was observed in 0 to 13 assays performed before 1 year of age, 3 of 8 assays performed between 1 and 4 years of age, and 9 of 15 assays performed over 4 years of age. In contrast, a positive LTR to HSV-1 was seen in 15 of 18 assays performed in children under 1 year of age and in 14 of 16 assays performed in survivors of neonatal HSV infection older than 1 year. Six HSV-2-infected patients were tested simultaneously 13 times with HSV-1 and HSV-2 antigens. Those patients under 6 months of age responded similarly to each antigen, whereas those who were older had significantly higher LTRs to HSV-2. Children with CMV infection that was acquired early had persistently diminished specific LTRs. In contrast, after neonatal HSV infection, LTRs to HSV were present even in infancy and became more specific for the infecting type with increasing age.