The histochemical NBT test was performed on blood samples from ten healthy newborn infants. High spontaneous NBT reduction has been confirmed for neutrophils and assessed for monocytes. The stimulation of both neutrophils and monocytes with Escherichia coli endotoxin induces a statistically significant increase of NBT positive cells. The reason for the false positive test results in neonates was investigated by incubating neutrophils from adult donors and for different time periods in either neonatal or adult plasma before the addition of NBT. NBT reduction by adult neutrophils was increased after incubation in neonatal plasma, and this increase was related to the concentration of the plasma used. Maximum NBT reduction was observed after 90 min of incubation, at which time the NBT scores of adult cells incubated in neonatal plasma were similar to the results of tests performed on whole neonatal blood. It is concluded that neonatal leucocytes demonstrate efficient spontaneous and stimulated phagocytosis, and that there are, in the plasma of neonates, humoral factors which stimulated phagocytosis by neutrophils and are thus responsible for the false positive NBT test results observed in these subjects.