Polymyxin B, which is a basic polypeptide produced by various strains of Bacillus Polymyxa, has previously been shown to prevent the lethal effect of LPS and to neutralize the Schwartzmann reaction. In this study we have investigated the interactions between polymyxin B and lipopolysaccharide (LPS) and hapten LPS conjugates. Polymyxin B was found to suppress mitogenicity of LPS and also to inhibit immunogenicity of the hapten conjugate 4-hydroxy-3,5-dinitrophenacetyl (NNP)-LPS. Inhibition was not due to interference with the expression of NNP determinants nor to cross-reactivity between PB and the hapten. Since mitogenicity and immunogenicity decreased in parallel, we conclude that B-cell activation in specific thymus independent responses does not take place in the absence of a nonspecific (non-Ig-mediated) signal.