We isolated a T8+ T3+ Ia+ clone of cells from the peripheral blood mononuclear cells of a healthy subject. The clone was expanded and maintained with autologous feeder cells, interleukin 2, and a streptococcal antigen. The T8+ clone of cells responded specifically to the streptococcal antigen, in the absence of accessory cells, and released a soluble factor. Both the cloned cells and the corresponding soluble factor expressed augmenting helper but not suppressor activity. The augmenting helper activity for B cell antibody synthesis was demonstrable only in the presence of autologous T4 cells. Although stimulation of the T8+ cloned cells was antigen-specific, the resulting soluble factor elicited nonspecific antibody synthesis in the presence of T4 and B cells. The T8+ cloned cell-derived factor was adsorbed by B cells but not by T4 cells. Preliminary studies suggest that the factor has the properties of a B cell growth factor. We suggest that the T8+ population consists of functionally heterogeneous cell subsets, some that have suppressor function and others that augment the T4+ helper-inducer activity in B cell antibody synthesis.