Foxp3(+) regulatory T cells (Tregs) are crucial for maintaining tolerance to self-antigens and preventing autoimmune diseases. Loss of Foxp3 expression leads to autoimmunity and disrupts humoral immune responses, including hyperproduction of immunoglobulin E (IgE). Elucidation of how Tregs control antibody production can lead to the development of new therapies for autoimmune and allergic diseases. However, premature death of Foxp3-deficient mice makes it difficult to analyze the roles of Tregs in humoral immunity of adult mice. In this study, we developed Foxp3 conditional-knockout mice (Foxp3(flox) R6(CreERT2)) in which the Foxp3 gene was inducibly deleted by tamoxifen administration. After oral administration of tamoxifen, titers of immunoglobulins, particularly IgG2c and IgE, were increased in Foxp3(flox) R26(CreERT2) mice compared with that in controls. Under these conditions, CD4(+) T cells from Foxp3(flox) R26(CreERT2) mice had increased expression of several activation markers, including inducible costimulator and CD40 ligand, as well as the cytokines interleukin 4 and interferon gamma. In addition, the proportions of T follicular helper (Tfh) cells and germinal center (GC) B cells were increased in Foxp3(flox) R26(CreERT2) mice compared with those in controls. These results indicated that Tregs controlled excessive or pathogenic antibody production by suppressing Tfh cell differentiation and GC formation. Furthermore, these data suggested that Foxp3(flox) R26(CreERT2) mice could be a useful tool for screening therapeutic agents. (C) 2019 Elsevier Inc. All rights reserved.