The differentiation of human B lymphocytes in vitro into Ig secreting cells, as measured by a radioimmunoassay rendered specific to IgG and IgM, in response to a polyclonal stimulant, requires the participation of several cellular types. Upon poleweed-mitogen stimulation, B-cell enriched and monocyte containing cell suspensions prepared by elution of nylon wool adherent cells, when mixed with various concentrations of T cells purified by filtration through nylon wool column and depletion of EAC-rosette forming cells, synthesize 5 to 20 times more Ig than non separated lymphocytes. These experiments which demonstrate the cooperation of certain T cells also suggest that other cells eliminated by nylon wool adherence act as suppressor cells. The cooperation of these T cells is independent of their proliferation and can be obtained with allogeneic T cell.