Various theories have been proposed to explain the mechanism of collaboration between thymus-derived (T) 1 and nonthymus-derived (B) lymphocytes in the induction of antibody responses. Among these is the possibility that T cells activated by antigen secrete a soluble factor capable of facilitating the induction of antibody production by B cells. In a system involving allogeneic combinations of lymphoid cells, Dutton et al. (1) in vitro and Katz et al. (2) and Kreth and Williamson (3) in vivo have obtained evidence for the release by T cells of a nonspecific factor stimulating B cell responsiveness. In these experiments T cells were activated by histocompatibility antigens, whereas the response of B cells to other antigens, such as heterologous erythrocytes or hapten-protein conjugates, was assayed. The recent demonstration by Schimpl and Wecker (4) of the production by alloantigenically stimulated T cells of a factor which could partially replace T cell function in the sheep red cell response has provided more direct evidence for the existence of a nonspecific component in T cell helper activity.In contrast, Feldmann and Basten (5, 6) have found that T cells activated by a particular antigen elaborated a factor capable of triggering syngeneic B cells, but only