This study of the effect of serum dialysis on thymidine incorporation into lectin-activated blood lymphocytes cultured in vitro showed that (1) after serum dialysis 3H-thymidine incorporation is very low or absent; (2) the addition of L-glutamine to dialyzed sera leads to recovery of 18% of 3H-thymidine uptake, and (3) addition of L-glutamine reduces the difference in the stimulating effects of acromegalic and hypopituitary sera observed before dialysis in a medium free of L-glutamine. Since the plasma level of glutamine is increased in acromegalics and reduced in hypopituitary patients, it is suggested that this amino acid acts as a growth hormone dependent factor or cofactor in the regulation of cell growth.