In this study the phagocytic and natural killer cell (NK) functions in 17 premature and 30 mature newborns are compared. The ability of polymorphonuclear phagocytes (PMNs) to ingest, digest and lyse (antibody-dependent cell-mediated cytotoxicity, ADCC) opsonized sheep red blood cells and NK activity were tested. Examinations were performed in cord and venous blood within 6 h or 3–4 days after delivery. Results of examinations were compared with normal values for the group of healthy 4- to 15-year-old children. To assess the influence of the newborn’s maturity and age on the tested PMNs and NK functions, the following comparisons were made. (1) Cord vs. peripheral venous blood: only ADCC was higher in peripheral than in cord blood. (2) Mature newborn cells obtained either 6 h or 3–4 days after delivery: ingestion and ADCC were lower and NK activity was higher 3–4 days after delivery. (3) Premature vs. mature newborn cells tested 3–4 days after delivery: ingestion and ADCC were higher while NK activity was lower in premature newborns. (4) Premature newborns tested at 3–4 days vs. mature newborns tested within 6 h after birth: ingestion was lower in the prematures while digestion, ADCC and NK activity were similar. (5) Cells from all newborns tested vs. those of healthy older children: results depend on the interval after birth when newborns were tested. Thus, within the first 6 h after delivery, mature newborns had higher ingestion and ADCC capacity but lower digestion and NK activity. Later, 3–4 days after birth, ingestion, ADCC and NK activity were lower in mature newborns. In the prematures at that interval NK activity was lower. (6) There was a positive correlation between gestational age and NK activity of newborns.