Neonatal effects of transplacental exposure to polychlorinated biphenyls (PCBs) and dichlorodiphenyl dichloroethene (DDE) were examined in a study of 912 infants. Birth weight, head circumference, and neonatal jaundice showed no relationship to PCBs or DDE. We also administered the Brazelton Neonatal Behavioral Assessment Scales, which are psychologic and neurologic tests designed for use in newborn infants. The results of these tests showed that higher PCB levels were associated with hypotonicity and hyporeflexia and that higher DDE levels were associated with hyporeflexia.