High-affinity IL-2R signals are required for peripheral lymphoid homeostasis in vivo. We found that CD25 was required for regulation of peripheral T cells in mice bearing either the DO11.10 MHC class II-restricted TCR transgene or an Iabeta-null mutation, suggesting that MHC class I- and class II-dependent T cell subsets are regulated independently by IL-2R signals. In contrast, deregulation of serum IgG1 levels in CD25-/- mice was dependent on CD4+ T cells. T cell expansion in DO11.10 CD25-/- mice was not preferential for cells escaping allelic exclusion by the TCR transgene, but was suppressed by a Rag-2-null mutation. Together, these findings suggest that endogenous TCR are required to trigger T cell expansion, but that CD25 regulates T cells activated by low-specificity signals. Expansion of DO11.10 T cells in response to cognate Ag was modestly reduced in CD25-/- T cells transferred into the normal lymphoid compartments of BALB/c mice. Moreover, activation-induced clonal contraction and apoptosis in vivo were intact in the absence of CD25. These data indicate that the regulatory role of high-affinity IL-2R signals extends beyond the control of Ag-specific responses and suggest a role for these signals in control of bystander T cell activation.