The scores of five cytochemical leukocyte reactions (myeloperoxydases, leukocyte alkaline phosphatases, nonspecific esterases, cell polysaccharides and Soudan black) were established in a group of 100 healthy neonates from normal pregnancies. Comparisons between infant and adult findings show that leukocyte alkaline phosphatase activity is distinctly higher in infants whereas that of mieloperoxydases and of lymphocyte cell polysaccharides is lower. The interpretation of data remains difficult. Little has been reported elsewhere concerning this type of investigation. Only leukocyte alkaline phosphatase reaction was found to have been studied systematically. Data are similar in this case. Such a study is interesting for the investigation of cell defense mechanisms against infectious agents, of genetic disease and for determination of normal values permitting discrimination between physiologic variations found in different age groups and pathologic variations found in different age groups and pathologic states.