One of the greatest challenges that has confronted the cellular biologist for the past decade has been the understanding of the basic mechanisms that underlie the interaction of antigen with the cell surface and the subsequent cell-to-cell interaction that follows through a chain of events to what constitutes an immune response. The dialogue of the cell-surface membrane between antigen and cell-surface recognition structures is of fundamental importance in the determination of the pathway that leads through a chain of precise, genetically regulated steps of differentiation. This insight of the molecular level has provided the basis for our current thinking. It demonstrates that the cell membrane surfaces are genetically defined and regulated and that immunobiologic mechanisms are fundamentally interconnected. The membrane and its structure serve a very important function, both in terms of conducting biological dialogue at the cell surface and the translation of this dialogue to physiological events that lead to limiting, suppressing, or controlling an immune response, as opposed to permitting, inducing, or overwhelming an immune response. These concepts are aligned with the Burnet's clonal selection theory of immunity. One of the inevitable assumptions of this theory is that heterogeneity of lymphocytes exists at the level of cell-surface antigen receptors. Similarly, the variety of responses, both cell-mediated and humoral, against a specific antigen implies that functional heterogeneity of lymphocytes also exists. Thus, an understanding of the immune system should incorporate those features of lymphocytes which govern immune recognition and those that relate to subsequent immune function. Over the past decade, evidence has been accumulating that there is considerable heterogeneity among lymphoid cell subpopulations. Our current understanding of this degree of heterogeneity largely stems from the discovery of cell-surface markers which are unique to subpopulations of lymphoid cells and the observation that certain specific immunologically mediated events are predominantly, if not exclusively, expressed by some lymphoid cells but not others. These studies have been further extended by the description of the sequential ontogenetic appearance of the cell-surface markers as relation to the acquisition of immune competence. The purpose of this review is to summarize our current knowledge of the cell-surface markers that are present on lymphoid cells.