Antibody-dependent, phytohaemagglutinin-induced, and spontaneous cytotoxicity was studied in 44 term and 60 preterm newborn babies, all of whom were healthy and of normal weight for gestational age. Twenty-seven adults were used as controls. Antibody-dependent cytotoxicity was low in term babies particularly in the preterm ones during the first 4 days of life, but soon rose to adult levels. Phytohaemagglutinin-induced cytotoxicity was low both in term and preterm babies compared with adult levels, and remained lower throughout the neonatal period although it began to rise. Spontaneous cytotoxicity was lower in preterm babies than in term ones during the first 2 weeks of life, and lower too than in adults. These findings indicate decreased cytotoxic ability of neonatal leucocytes especially during the first 4 days of postnatal life particularly in preterm babies, suggesting either lack of effector cells or that the cells are functionally immature.