Immunoglobulin J chain mediates the polymerization of both IgM and IgA immunoglobulins. Its synthesis is closely regulated in B lymphocytes, apparently at the level of RNA transcription. To define the genetic bases of this regulation, we have determined the location and number of J chain genes in the mouse. Analysis of DNA from a group of somatic cell hybrids containing various mouse chromosomes on a constant background of Chinese hamster chromosomes indicated that this gene is located on mouse chromosome 5, unlinked to immunoglobulin heavy and light chain structural genes. Restriction mapping experiments further suggested the existence of a single J chain gene per haploid genome. This result was confirmed by quantitative analyses of band intensities yielded by Southern blots of mouse genomic DNA and J gene-containing plasmid DNA.