Thirty outbred New Zealand White rabbits were hyperimmunized i.v. with type II pneumococcal vaccine. Whole antisera were analyzed by isoelectric focusing (IEF) in the pH range of 5 to 9. Dissimilar IEF patterns were recorded for most of them with the exception of two rabbit antisera, 4422 and 311, which exhibited two antibody components with an indistinguishable IEF pattern. The antiserum 4422 was highly electrophoretically restricted, whereas antiserum 311 displayed several antibody components. Antibody components from antiserum 311 were isolated by affinity chromatography and the fraction exhibiting an IEF pattern indistinguishable from that of antibody 4422 was found to bind a nonasaccharide hapten derived from SII pneumococcal polysaccharide with the same affinity as antibody 4422. Both antibodies expressed a1/b4 allotypic specificities. Crossrecombinant molecules between antibodies 4422 and 311 exhibited the same affinity constant as that of parent antibody molecules, thus strongly suggesting that these have the same antigen complementarity-determining regions. Anti-idiotypic sera were prepared in guinea pigs against both antibodies. The reactions between 125I-labeled antibodies and their corresponding absorbed antiidiotypic serum were inhibited to the same extent by unlabeled antibody 4422 or 311, an observation that is consistent with the expression of full idiotypic crossreactivity between this pair. No inhibition was obtained with four other anti-SII antibodies of IEF patterns different from those of 4422 or 311. The guinea pig antiidiotypic sera were not antigen-binding site specific since the nonasaccharide hapten was not inhibitory in this system. Taken together, the results strongly suggest a high degree of structural similarity of two antibodies from outbred rabbits.