A cloned DNA segment 1.25 kilobases (kb) upstream from the joining segments of the human heavy chain immunoglobulin gene revealed extensive polymorphic variation at this locus, and the polymorphic pattern was stably transmitted to the next generation. Genomic restriction analysis showed that the polymorphism was caused by insertions/deletions within an MspI/BamHI fragment. Sequencing of one allele, 848 base pairs (bp) long, revealed eleven 50-base-pair tandem repeats. A second allele, 648 bp long, was cloned from a human genomic cosmid library, sequenced, and found to contain four fewer repeats than the first allele. A survey of 186 chromosomes from unrelated individuals of primarily northern European descent revealed at least six alleles.