Mice immunized with a combination of dextran B1355 in adjuvant followed by three injections of 2 x 109 Escherichia coli B organisms produced an average of 14.5 mg/ml of anti dextran antibodies. It was demonstrated that the stimulating effect of E. coli B was due to antigenic determinants cross reactive with B1355 and not solely because of adjuvant properties of the organism. The anti dextran antibodies were distributed among both 7S and 19S components. Isoelectric focusing of the 7S antibodies showed several spectrotypes of antibody, most of which were shared by the majority of the individual sera. The limited spectrotypic heterogeneity of the 7S antibodies was supported by idiotypic studies. Thus, a heterologous, anti idiotypic serum, rabbit anti M104, was prepared which distinguished between two closely related myeloma proteins, M104 and J558, with specificity for α (1 → 3) dextran. This antiserum demonstrated that some, but not all, of the 7S and 19S anti dextran antibodies possessed variable region determinants cross reactive with M104.