The monoclonal antibodies produced by a variety of hybridomas making antibody specific for E. coli 0113 lipopolysaccharide (LPS) were purified by affinity chromatography and their fine specificity studied. All reacted specifically with the polysaccharide moiety of LPS from E. coli 0113 and from Neisseria lactamica; two reacted with LPS from Pseudomonas aeruginosa and one reacted with LPS from Klebsiella pneumoniae. Polyclonal and monoclonal syngeneic and semi-syngeneic anti-idiotypic antisera were produced to study the idiotypy of LPS-specific monoclonal antibodies which express a complex cross-reactive idiotype (IdX) as well as individual idiotypes. E. coli 0113 LPS-specific antibodies produced by BALB/c mice express this IdX and the kinetics for its expression was examined using mice either primed or hyperimmunized with LPS; idiotypic maturation was observed, but we were unable to detect an auto-anti-idiotypic antibody response. This IdX was expressed on E. coli 0113 LPS-specific antibodies from all strains of mice examined, indicating that its expression is not restricted by genes linked to the IgCH locus.