Hapten-carrier cooperation reflects cellular cooperation in the induction of immunity. Convincing arguments in favour of this hypothesis are at hand. One can therefore apply hapten-carrier systems to the study of the mechanism of cellular cooperation. The experiments favour the view that the function of the thymus-derived helper cells is antigen-mediated stimulation of antibody forming cell precursors, which are predetermined for the class and the specificity of the emerging antibodies. The importance of the helper cells for the regulation of the immune response is evident in the induction of both immunity and paralysis. Paralysis, in particular, appears to be localized in most instances in the population of the helper cells. The function of the helper cells in the induction of antibodies seems to be preselection of antigens and subsequent amplification of the immune response.