To characterize the serum growth promoting activity estimated by means of bioassay (Thymidine Activity), the gel filtration chromatographic profile of sera obtained from 10 normal adults and from 10 normal infants was performed. For each serum fraction we evaluated the capacity to stimulate the tritiated thymidine (3H-TdR) uptake into lectin-activated lymphocytes. This capacity was expressed as percent increase in 3H-TdR uptake in presence and in absence of the optimal dose (1.25%) of the fraction. The highest percent increase in 3H-TdR uptake was obtained in the same three fractions (1500-2500 Kd, 28-42 Kd, 1.3-1.9 Kd) in the sera of adults as well as infants. The values observed in infants were higher than in adults, suggesting that the serum growth-promoting activity could be higher in rapidly growing subjects.