The kinetics of the response to passively transferred maternal neutralizing antibody were studied for determination of whether this antibody offered protection against primary cytomegalovirus (CMV) infection in the offspring. Antibody response was determined in 17 infants infected in the immediate perinatal period and in 18 appropriate control subjects. Levels of neutralizing antibody in serum obtained at birth and at one month of age were similar in infected and in exposed, uninfected neonates. In addition, the quantity of neutralizing antibody did not influence the time of onset of viruria. Clearly, passive humoral immunity failed to prevent naturally acquired primary CMV infection in a significant number of young infants exposed to virus during and shortly after delivery.