The murine immune response to a haptenated lipopolysaccharide (LPS) lacking repeating oligosaccharide determinants was studied. The LPS was extracted from a rough strain of bacteria (Salmonella minnesota R595) and chemically haptenated with either trinitrophenol or fluorescein isothiocyanate. These preparations of hapten-R595 LPS were shown to be immunogenic. Furthermore, the immune response to the hapten was demonstrated to occur independent of T cells and was not merely the result of enhanced polyclonal B-cell activation. The capacity of such hapten-LPS conjugates without repeating polymeric structures to stimulate T-independent antibody responses provides information on the molecular requirements for the activation of murine B lymphocytes.