The age-related variations in the growth-promoting activity of human plasma have been studied from birth (cord blood) to adulthood using a bioassay which measures the incorporation of tritiated thymidine into lectin-activated human lymphocytes. Cord blood values were low (0.69 +/- 0.004 U/ml). A definite increase was found at 5 days of age, correlating with the level at birth. Higher levels were attained after 1 month of age, with a 2-fold increase during the first months of life. Lower values were found in children 1-10 yr old, and high levels were found during puberty. This pattern, different from those of sulfation activity and plasma somatomedins suggests that factors other than somatomedins may be involved in growth stimulation during the first year of life in humans.