Spleen cells from C3H/HeJ mice fail to respond with polyclonal antibody synthesis to mitogenic concentrations of lipopolysaccharide (LPS) which are optimal for activating spleen cells from a high-responder strain (B10.5M). This unresponsiveness is selective for LPS, since C3H/HeJ cells respond as normals to another B-cell mitogen, purified protein derivative of tuberculin. Spleen cells from low-responder mice also fail to mount a specific anti-NNP plaque-forming cell (PFC) response, when challenged in vitro by NNP-LPS. However, C3H/HeJ cells develop normal responses to another thymus-independent hapten conjugate, DNP-AECM-Ficoll. C3H/HeJ mice fail to mount a specific anti-LPS antibody response, when challenged in vivo with doses of soluble LPS which are fully immunogenic for the high-responder strain. However, C3H/HeJ mice develop normal direct and indirect PFC responses to LPS, when challenged with a thymus-dependent form of the immunogen. These results are interpreted as indicating as absolute requirement for functional mitogenicity of the antigen, in the induction of specific thymus-independent antibody responses.