To study the lack of fever during the human newborn period, cord blood leukocytes obtained at birth were stimulated to produce leukocytic pyrogen (LP) in vitro. Phagocytic leukocytes from infants who were born by Caesarean section and whose mothers had not experienced natural onset of labor produced no LP or significantly less LP than leukocytes from adults or from infants born after natural onset of labor. There was a significant difference between total white blood cell count in cord blood of infants whose phagocytic cells produced LP and those whose cells did not. This observation could not be accounted for by anesthetic agents, phagocytosis of staphylococci, or number of leukocytes producing LP; thus, they suggest an intrinsic defect in the ability to produce LP before birth. Of interest is that a nondialyzable substance(s) present in crude preparations of human chorionic gonadotropin markedly suppressed LP production from adult human monocytes, but purified human chorionic gonadotropin had no effect.