We have assessed the number of times the gene sequence encoding constant regions of mouse immunoglobulin heavy chains gamma1, gamma2a, and gamma3 are represented in the mouse genome by hybridization kinetic analysis. All three genes are present at one copy each per haploid genome in normal tissues and myelomas producing IgM or IgG3. IgG1-producing myelomas, however, contain 1 copy each of the gamma1 and gamma2a genes and 0.5 copy of the gamma3 gene per haploid genome. IgG2b-producing myelomas contain 1 copy of the gamma2a gene and 0.5 copy each of the gamma1 and gamma3 genes per haploid genome. IgG2a-producing myelomas contain 1 copy of the gamma2a gene and 0.5 copy each of the gamma1 and gamma3 genes per haploid genome. In myelomas producing IgA, all three gamma genes are represented 0.5 times per haploid genome. In order to account for the results we propose an allelic deletion model: (i) The specific deletion of heavy chain constant region genes accompanies the recombination of a variable region gene to a constant region gene. (ii) The portion of the chromosome that resides between two joining sequences is excised out of the chromosome. (iii) The recombination occurs on one of the alleles. Based on this model we also propose that heavy chain genes are arranged on one chromosome in the following order; variable region genes, unknown spacer sequence, mu, gamma3, gamma1, gamma2b, gamma2a, and alpha.