Ig heavy chain class switch recombination (CSR) involves a recombination/deletion mechanism that exchanges the expressed C(H) gene with a downstream C(H) gene. CSR is mediated by highly repetitive switch (S) region sequences and requires the activation-induced deaminase (AID). The S region 5' of the C mu gene (S mu) can undergo high-frequency internal deletions in normal B cells and B cell lines activated for CSR, although the relationship of these deletions and CSR has not been elucidated. In this study, we introduced constitutively transcribed S mu or S gamma 2b regions into a pro-B cell line that can be activated for AID expression, CSR, and endogenous S mu deletions. We find that randomly integrated S region transcription units in these cells also undergo increased levels of internal rearrangements after cellular activation, indicating that the deletion process is independent of location within the Ig heavy chain locus and potentially AID-promoted. To test the latter issue, we generated hybridomas from wild-type and AID-deficient activated B cells and assayed them for internal S mu deletions and S region mutations. These studies demonstrated that efficient intra-S region recombination depends on AID expression and that internal S region deletions are accompanied by frequent mutations, indicating that most S region deletions occur by the same mechanism as CSR.