T-cell maturation has traditionally been felt to occur primarily within the thymus but it is now clear that dynamic processes in the periphery govern many functions such as T-cell activation, proliferation, tolerization, and migration into peripheral tissues. Four T-cell growth factors have now been identified. These include: interleukin-2 (IL-2), IL-4, IL-7, and a potent cofactor recently described, IL-10. These factors are believed to work synergistically in the fine regulation of the lymphoid pool. Both IL-2 and IL-4 have entered clinical trials with significant responses in the IL-2-based regimens of up to 40 to 50% in certain tumors. IL-7 and IL-10 are in preclinical studies. Although IL-2, IL-4, and IL-7 have been shown to induce lymphokine-activated killer cell activity from sensitive precursors, such studies have yet to be performed with IL-10.