Cell-mediated immunity (CMI) to cytomegalovirus (CMV) and herpes simplex virus type 1 (HSV-1) was examined in 35 mothers and 30 of their offspring with congenital or neonatal CMV infection by means of the lymphocyte transformation assay. Eleven offspring did not respond to CMV antigen, and 15 of the 19 positive children displayed lower responses than those of normal immune adults. Productive infection in the younger children at the time of assay and the presence of disease correlated strongly with the absence of responses. The mothers as a group also demonstrated impaired CMI to CMV while reacting normally to HSV-1 antigen. Neither time after transmission of infection nor viral excretion was significantly associated with impaired CMI, although a trend toward diminished responses was evident among women who were shedding virus. Mitogen stimulation was normal in all test subjects. These findings imply that deficient specific CMI may play a role in the genesis of persistent CMV infection and fetal pathology.