In the last ten years, considerable progress has been made in our understanding of the physiology of the lymphoid system. These include advances in knowledge of the biochemical nature of the receptors used by B and T lymphocytes in antigen recognition, the origin of the diversity of specificity exhibited by these receptors, and the crucial role played by growth factors in the cellular proliferation elicited during immune response. Moreover, the biological significance of the existence of two subpopulations of T lymphocytes has been reinterpreted in the light of recent studies showing a correlation between expression of the surface markers T4 or T8, and the restriction pattern of the antigenic specificity exhibited by lymphocytes bearing these markers.