Gene clones encoding the human J chain, the protein that links immunoglobulin monomers, were recently described. Using probes from J chain clones we have now investigated the chromosomal location of this gene by analysis of somatic cell hybrids and by in situ chromosome hybridization. The gene is located on the long arm of chromosome 4 in band q21, the chromosomal band in which a consistent translocation with chromosome 11 has been observed in some acute leukemias. An additional human sequence that cross-hybridizes with some J chain gene probes is located on a different chromosome. Restriction fragment length polymorphisms deriving from length variations in a tandemly repeated region 5' to the J chain gene were detected; these should facilitate the analysis of genetic linkage between this gene and other markers on chromosome 4.