SUMMARYAntigen‐specific T‐cell helper factors were secreted from a (T,G)‐A—L specific T‐cell line and clones. The factors were released upon antigenic stimulation and could be induced by a low or a high dose of antigen. The factors secreted upon low‐dose stimulation possessed the antigenic specificity of the secreting cells, while the high dose‐induced factors had a broader antigenic specificity and could react with the closely related polypeptide (Phe,G)‐A—L, even when the cells were restricted to (T,G)‐A—L. Both the low dose‐ as well as the high dose‐induced factors could not trigger antibody production in the presence of a non‐relevant antigen, and did not collaborate with B cells immunized with a non‐related antigen for the production of antibodies. The helper factors, like their secreting cells, wereH‐2‐restricted in the collaboration with B cells. In contrast to the helper cells, however, they did not require accessory cells for triggering the B cells in the process of antibody production. Some preparations of helper factors were found to be inactive. The helper activity could be restored by IL‐2. Thus, IL‐2 is an additional essential factor required for the antigen‐specific collaborati