Activation of T lymphocytes is initiated by receptor ligand interactions at the cell surface leading to the transduction of intracellular signals followed by the de novo synthesis and expression of T cell activation markers (including receptors for interleukin 2 (IL 2) and transferrin), production of lymphokines, and T cell proliferation. This requisite first step for activation of T lymphocytes can be mimicked in certain situations with a variety of stimuli. These include antibodies to certain integral membrane proteins, phorbol esters, and plant lectins that act as mitogens. In this paper, we report that at least two classes of human T cell clones can be distinguished based upon signal requirements necessary to induce proliferation. Although all clones analyzed expressed IL 2 receptors and secreted IL 2 after non-antigenic activation, one subset of clones did not proliferate in response to the same non-antigenic signals. In that subset, complete activation leading to proliferation required interaction of the T cell with specific antigen. The ability to subset these T cell clones into two groups did not correlate with phenotypic differences, source of the clone, nor with magnitude of intracellular calcium mobilization. By studying the stimulation requirements of these two subsets of human T cell clones through the use of specific antigen or antigen-independent stimuli, it was possible to demonstrate that different stimuli varied in their ability to induce steps of T cell activation. Analysis of reactivity of these clones to suboptimal stimulation allowed the definition of intermediate stages of T cell activation. Such intermediate stages might reflect a diversity of intracellular signaling pathways or a complexity of regulatory mechanisms distal to the events that allow intracellular calcium mobilization. Thus for the first time, it has been possible to study ordered events of T cell activation in non-transformed, antigen-dependent human T lymphocytes. The data presented in this paper suggest that T cell activation is not an all or nothing phenomenon, and there is an ordered sequence of events that can be differentiated based upon signal requirements at the T cell membrane.