Monoclonal antibodies capable of exerting agonistic functions toward surface receptors of human T lymphocytes are employed to study discrete signals involved in T cell activation via two differential pathways (antigen-induced and "alternative" pathway of T cell activation). The studies to be reported demonstrate that these two modes of T cell triggering are independent of each other since the respective cell surface molecules with which T cells interact in the initiation of activation and the requirements for monokines are clearly distinct. Moreover, preliminary investigations of various types of secondary immunodeficiencies suggest that immunologic dysfunction can be attributed to blockades at particular levels of the activation cascade.