This chapter provides an overview of antigen-specific factors, which are produced by T-lymphocytes. Antigen-specific factors may be divided into two broad categories: antigen-specific helper factors (THF), produced by T-helper (T-H) cells; and antigen-specific suppressor factors (TSF) produced by T-suppressor (T-S) cells. It has proven to be easier to obtain T lymphocyte clones or T lymphocyte hybridomas which release TSF than it has been to establish T cell clones or T cell hybridomas which make antigen-specific THF. Most of the T-H clones or T-H hybridomas produced so far have shown a requirement for specific antigen for activation but release the non-antigen-specific interleukin 2 (IL-2) (or T cell growth factor) rather than antigen-specific THF. It should also be noted that THF and TSF which display antigen specificity may be idiotype-binding rather than antigen binding in some experimental systems. Thus, it is possible to have either idiotype- or antigen specific THF or TSF acting to modify the response to a given antigen. © 1985, Elsevier Inc. All rights reserved.