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A population of IgA molecules having heavy chains coded by two parental chromosomes in trans position has been identified in rabbits heterozygous at both the V(H)a locus, which controls allotypic specificities on the variable part of heavy chains, and the C(alpha)g locus, which controls allotypic specificities on the constant part of alpha chains. These recombinant molecules have alpha-chain allotypic specificities controlled by both the maternal V(H)a gene and the paternal C(alpha)g gene or conversely, the paternal V(H)a gene and the maternal C(alpha)g gene. These recombinant molecules were found in F(ab)(2alpha) fractions obtained after passage of F(ab)(2alpha) preparations through immunosorbent columns designed to remove one population of F(ab)(2alpha) molecules, i.e., g74- or g75-type molecules. The effluent F(ab)(2alpha) fractions were then examined by radioprecipitation methods for allotypic specificities controlled by the V(H)a and C(alpha)g loci. About 40% of the g75 F(ab)(2alpha) molecules from each of three rabbits with the a(1)g(74) and a(2)g(75) allogroups were alg75 recombinants. These alg75 recombinant molecules represented from 2.5-5.6% of the total unfractionated F(ab)(2alpha) sample. The F(ab)(2alpha) fractions from two rabbits with the a(1)g(75) and a(3)g(74) allogroups had from 1.8-8.2% recombinant molecules: some were alg74 recombinants and some were a3g75 recombinants. Somatic recombination as a mechanism responsible for the synthesis of polypeptide chains in which part of the information is obtained from one chromosome and part from the homologous chromosome is discussed.