A series of seven monoclonal antibodies was produced against the T-lineage-specific 50 kd T11 sheep erythrocyte rosette (SRBC) receptor protein in order to define the function of the molecule. Three distinct epitopes were detected: T11(1), the SRBC binding site expressed on all T lymphocytes and thymocytes; T11(2), an epitope unrelated to the SRBC binding site but with a similar distribution; and T11(3), a neo-epitope expressed only upon T-cell activation. Simultaneous triggering of T11(2) and T11(3) epitopes by monoclonal antibodies induces T lymphocytes to proliferate and mediate their functional programs in the absence of antigen and/or antigen-presenting cells. This antigen-independent mode of triggering is distinct from that involving the T3-Ti antigen receptor complex and represents an alternate pathway of T-cell activation. Given that T11 is the earliest T-lineage surface glycoprotein to appear in thymic ontogeny and is thus expressed before T3-Ti, the former may be involved in clonal expansion and/or differentiation during early development.