Previous investigations have shown that peripheral lymphocytes with a low specific gravity are much more active than are heavy lymphocytes in the proliferative response towards mitogens, antigens and allogeneic lymphocytes. We have now studied the biological significance of this difference in human peripheral T lymphocytes with two independent parameters generally regarded as maturation criteria. Heavy human T lymphocytes contain about one-third of the amount of cyclic AMP found in light T cells, but the increase in cyclic AMP produced by isoproterenol or prostaglandin E1 is much higher in the heavy T cells. The lactate dehydrogenase isoenzyme pattern of heavy human T lymphocytes is intermediate between that of human thymocytes and light T lymphocytes, the latter showing the most mature pattern. A high correlation was found between these two parameters, the cell density and the reactivity towards phytohemagglutinin. We conclude that human peripheral T lymphocytes are heterogeneous in maturation level: heavy, nonreactive T lymphocytes are less mature than light T lymphocytes.