Fatal pulmonary herpes simplex of the newborn was acquired during passage through the birth canal of a mother with primary herpetic vulvovaginitis. Administration of gamma globulin to the mother before delivery and to the infant after delivery failed to prevent the disease in the infant or to alter its fatal course. Vulvovaginitis in the mother did not appear to be altered. Gamma globulin given to mother and infant contained significant amounts of herpes-neutralizing antibody and low, yet easily detectable, titers were present in the infant's serum. The probability that premature and newborn infants will develop serious or fatal herpes simplex infection after exposure makes it necessary to isolate these infants from persons with known herpes simplex. In addition to large doses of gamma globulin, cesarian section should be considered in future instances of this type.