We have analyzed the configuration of the H chain locus of 41 hybridomas by Southern blot analysis. Each H chain switch region was determined to be germ line, rearranged, or deleted. Including 13 previously analyzed hybridomas, 60% of those with rearrangements on both alleles showed a correlation of the two alleles, i.e., both the expressed and the nonexpressed alleles have rearranged to the same H chain constant region gene segment. When the two H chain alleles did not rearrange to the same gene, they often rearranged to neighboring H chain genes. These results support a role for isotype-specific factors in H chain switch recombination. The action of these isotype-specific factors may be propagated to some extent along the chromosome, which would lead to rearrangements to neighboring genes.