Lysozyme levels were determined in serum and umbilical cord blood of 352 newborns and prematures. Levels in premature babies were found to be significantly lower than those of matures at the first day of life. A correlation was seen between the serum lysozyme and the birth weight of 219 mature newborns. In 14 premature babies with clinical signs of sepsis the concentrations of serum lysozyme were particularly decreased in cases of septicemia caused by gram-negative organisms. Serum levels of lysozyme in cord blood were significantly lower in 38 newborns with predisposition to septicemia (above all premature rupture of membranes greater than 24 hr.) comparing with healthy infants. The decreased serum levels of lysozyme in newborns with septicemia and the remarkable susceptibility of infections in male newborns are discussed.