Allospecific mouse T cells, positively selected in one-way mixed lymphocyte culture were maintained for 3 yr in tissue culture by sequential restimulation. Such proliferating T cells were tested for their ability to induce a positive allogeneic effect: activating B cells in an in vitro primary humoral response to sheep erythrocytes. It was found that such T lymphocytes could function as helper cells. Helper activity was shown to be specific in that the B cells activated had to share major histocompatibility complex (H-2) antigens with the strain used for selection of the cell line. Intra H-2 mapping showed that antigens coded in the IAk subregion played an important role in the induction of the positive allogeneic effect. Supernatant factors could substitute for the allogeneic T cells in activation of the in vitro humoral response. However, such supernates exhibited no strain specificity. Therefore, the specificity seen in the positive allogeneic effect is presumably a consequence of the alloantigenic recognition receptors intrinsic to the T cells, and not to any biologically restricting properties of the allogeneic effect factor itself.