The Ly-6 pathway of T cell activation was analyzed to identify its essential requirements. Using a monospecific chicken antiserum to Ly-6E, fully cross-reactive to its allelic counterpart, Ly-6A, but unreactive with other members of the Ly-6 family, we have found that interferon (IFN)-alpha/beta or gamma, Ly-6 antibody and interleukin 2 (IL 2) act synergistically in inducing T cell proliferation. The action of IFN can be attributed to induction of Ly-6A/E antigen on T cells, as described previously, and this induction is transcriptionally controlled. Exposure of T cells with elevated Ly-6 concentrations to chicken anti-Ly-6 antibody leads to expression of IL 2 receptors. Consequently, the addition of IL 2 drives T cell proliferation. Thus, in BALB/c mice the minimum requirements for activation by the Ly-6 pathway are IFN (as a means of inducing Ly-6). Ly-6 antibody (as inducer of IL 2 receptors) and IL 2. In mice of the Ly-6.2 haplotype, IFN is not an absolute requirement. This may be related to the fact that these animals, in contrast to those of Ly-6.1 haplotype, express Ly-6 constitutively on a substantial proportion of resting T cells. Thus, T cells of C57BL/6 or DBA/2 mice can be induced to proliferate with Ly-6 antibody and IL 2 alone, although IFN pretreatment enhances this response. In BALB/c mice the IL 2-driven proliferative response induced through the Ly-6 pathway occurs selectively in the L3T4- population.