Allogeneic immunizations between mice strains that differ at multiple loci coding for minor histocompatibility antigens (mha) result in a variety of immune responses, including the induction of cytotoxic T cells, helper T cells, and delayed hypersensitivity reactions. However, high-dose intravenous immunization induces a population of cells that can suppress these responses. In this paper, we show that the transfer of this suppressor population in vivo is accompanied by a reduced ability of immune cells to produce the two lymphokines interleukin-2 (IL-2) and interleukin-3 (IL-3). In a different assay of suppressor function, suppressor cells were co-cultured with responder cells in vitro. Under these conditions, the presence of suppressor cells resulted in a lowered net production of IL-2, but not IL-3. Possible mechanisms for this phenomenon are discussed.