Lymphocyte subpopulations responsible for immunologic memory developing within 4 days of priming with small numbers of sheep erythrocytes (2 × 106 SRBC, i.v.) were investigated by transfer of spleen cells into irradiated syngeneic mice, challenge with SRBC or SRBC-TNP conjugate, and enumeration of plaque-forming cells at the time of peak response. Cell suspensions were enriched with T and depleted of B lymphocytes, or vice versa, by filtration through nylon wool columns and separation of nonadherent from adherent cells. The characteristics of anti-SRBC or anti-TNP responses by primed unseparated cells (i.e., acceleration, augmentation, increased IgG:IgM ratio) were retained when T-rich spleen cells from primed mice were reconstituted with B-rich cells, irrespective of whether the latter were from primed or unprimed donors. Responses by unprimed T cells in combination with primed B cells resembled those elicited by unprimed unseparated spleen. Hence, low doses of antigen selectively activated the T lymphocyte subclass; helper cells became more effective after priming for cooperative interaction with B cells, especially with those committed to IgG production. Under conditions of limiting dilution for T cells, the helper units of primed mice (i.e., functional units of helpers and amplifiers) were restricted to interact with either IgM or IgG precursor B cells, but not with both, since direct and indirect plaque responses assorted independently of each other. The immediate precursors of primed helper cells were peripheralized T lymphocytes, in keeping with the persistence of helper activity for months after adult thymectomy. The incompetence of large numbers of transferred thymocytes to generate such helper cells was in sharp contrast with their prompt generation by small numbers of T-rich spleen cells.