Regulatory T cells recovering after chemotherapy show clear oligoclonal skewing, suggesting that their proliferation is antigen-driven. TCR spectratyping analysis was used to assess the diversity of the recovering T lymphocytes, specifically the types and lengths of TCR expression. Unsorted T lymphocytes derived from healthy controls expressed a full range of Vβ chains with approximately normal Gaussian distributions of the lengths of the Vβ chains for each Vβ (< 3% driven peaks). Comparatively, unsorted T lymphocytes from a representative patient on day 21 after AcDVP16 showed a full complement of Vβ chains, but the lengths of these chains varied somewhat from a normal distribution (27.8% driven peaks). Conversely, regulatory T cells from that same patient on that same day expressed a markedly constricted range of TCR Vβ chains with striking oligoclonal skewing (75.6% driven peaks). These data suggest that the proliferation of regulatory T lymphocytes after timed sequential chemotherapy is antigen-driven.