Recombinations and hybridizations of H and L‐chains derived from several homogeneous rabbit antibodies to type 3 pneumococcal polysaccharide (SIII) were carried out. All reconstitution experiments performed gave rise to genuine IgG molecules. Antigen‐binding studies and affinity measurements for a hexasaccharide ligand derived from SIII were made. In addition, heterologous antiidiotypic serum raised against one rabbit anti‐SIII antibody was used to measure the reconstitution of idiotypic determinants in hybrid immunoglobulin molecules. The results show that full recovery of the antigen‐binding properties was obtained only when chains derived from the same antibody molecules were reassociated. Similarly, the complete regain of idiotypic determinants (studied in one antibody system) could only be demonstrated in the homologous recombinants. The pairing of an H‐chain with several heterologous L‐chains, which differed in 6–11 positions in the 3 hypervariable sections, led to the formation of hybrid IgG molecules which had an affinity at least 100‐fold lower than that of the parent antibody molecule and a number of hapten‐binding sites which did not exceed 0.30.