Cell transfer experiments were carried out with phytohaemagglutinin-induced normal human T lymphocyte colonies after 2--10 days of primary colony growth. The cells gave a cloning efficiency of 15% after 2 days of incubation and this decreased to 0.2% with the progressive growth of the colonies. The primary colonies had a proliferative capacity to give about 240 cells after 10 days of incubation, but only contained 0.5 to one cell per colony that could form a new colony. This number of colony-forming cells per colony did not change with an increase in colony size. These results indicate that colonies maintained the same number of colony-forming cells that they started with and that the other cells could proliferate but not form colonies. It is suggested that this ability to distinguish between colony-forming and proliferative T cells may be useful for determining specific deficiencie in either cell type in various diseases.