Characteristics of specific cell-mediated immunity in infants with congenital cytomegalovirus (CMV) infection were studied by in vitro lymphocyte transformation test, using the whole blood culture technique. Subjects were 6 symptomatic congenitally infected infants, 11 asymptomatic congenitally infected infants, 3 postnatally infected infants, and 5 CMV-free infants. Among them, three symptomatics and seven asymptomatics were studied temporally, together with three postnatally infected infants and five controls. In the congenitally infected group, 18 of 47 (38%) specimens proved to be negative in terms of stimulation index values. Studies of temporal changes in CMV lymphocyte transformation tests revealed that in the postnatally infected group, the response turned positive in accordance with the appearance of CMV viruria and remained positive afterwards. In the congenitally infected group, the delay in development of positive reaction followed initial negativity, a phenomenon conspicuously observed in the symptomatic subjects. No significant difference was seen in phytohemagglutinin-induced lymphocyte transformation tests among congenital, postnatal, and control groups.