The technique of genomic Southern blot analysis was employed to map the sites of class switch rearrangement in five hybridoma cell lines, four of which produce gamma 2b, whereas the fifth secretes gamma 2a. Two of the class switch recombinations we have mapped represent secondary switches to gamma 2b with a gamma 1 intermediate. We also observe isotype specificity in class switch recombination, with both allele rearranging to the same isotype in the three lines where an unexpressed chromosome is retained. Every 5' break occurred outside of and immediately 5' to the tandem repeats associated with Cmu, a result consistent with earlier studies on myelomas but in contrast to the pattern seen in lymphomas. Seven of the eight 3' switch sites, however, were within tandem repeats. This disparity in the distribution of 5' and 3' break points may reflect a unique role for the mu-associated tandem repeats in the switch process.