2,4-Dinitrophenyl hapten, when coupled to Salmonella choleraenuis bacilli (DNP-Salm), was capable of inducing sequential IgM and IgG antihapten antibody responses in the mouse, both of which were found to be totally thymus-independent. The antigen could also elicit a secondary antihapten antibody response of both IgM and IgG classes in mice preimmunized with the hapten coupled to either homologous or heterologous carrier. No ‘helper’ or ‘suppressor’ acitivity was found in salmonella-primed T cells. However, this antigen was not capable of inducing IgE antibody response even with the aid of adjuvants. Furthermore, the time-dependent increase in antibody affinity (maturation) did not occur following repeated injections of DNP-Salm.