The relationship among circulating values of growth hormone (GH), thymulin, and zinc in 19 healthy human neonates at birth and at the 4th month of age, and in their respective mothers, was investigated. Cytofluorimetric analysis on some CD antigen markers was conducted on cord blood and peripheral blood mononuclear cells. Active thymulin and zinc plasma levels increased in newborns in comparison with their mothers. In neonates serum GH levels increased with a significant decline later. The expression of CD molecules from newborns at birth and from infants at the 4th month of age was inversely correlated with active thymulin, zinc, and GH levels, whereas CD4 antigen marker was positively correlated with the same parameters at the 4th month of life. A novel interrelationship among active thymulin, zinc, and GH exists from the early up to the late phase of newborn life, in which maternal zinc, via lactation, may be involved.