A cloned helper T cell line, L2, has been derived that promotes the proliferation of cytolytic T cells with the concomitant expression of cytolytic activity. L2 helper cells also cause a polyclonal stimulation of B cells in the absence of antigen, as measured by the number of total plaque-forming cells (PFC) as well as those generated toward TNP or SRBC. The L2 helper cell function is dependent on cell number, is radioresistant, and is mediated via soluble factor(s). Maximum levels of PFC were generally observed after 4 days of culture, with IgM-secreting B cells being predominantly induced in these cultures. Both the proliferation of and stimulation by L2 helper cells are activated by Misa determinants. F1 progeny from a mating of CBA/N and DBA/2 (Mlsa) that did not bear Mlsa (NDF1) were nonstimulatory, whereas those F1 animals bearing Mlsa (DNF1) induced L2 helper-cell proliferation and function. L2 helper-cell recognition of Mlsa determinants present on the responding B cell surface was not required for polyclonal helper-dependent stimulation in vitro. L2 cells stimulated polyclonal antibody secretion in syngeneic as well as allogeneic B cells. Since Mls antigens are not expressed on T cells, these results indicate that regulatory T cell circuits are initiated by non-T cells, possibly B cells.