Follicular regulatory T (T) cells, a subset of Foxp3 regulatory T cells (Tregs), play an essential role in the regulation of germinal center (GC) response and humoral self-tolerance. Although it is generally accepted that T cells suppress GC antibody responses mediated by follicular helper T (T) cells and B cells, the helper activity of T cells on GC responses has also been recently reported. Because of this, it is critical to develop specific assays that are able to precisely assess T cell function, particularly its in vivo activity, independent of differentiation and other Tregs. Here we describe an adoptive transfer approach in conjugation with flow cytometry and ELISA to evaluate the T cell function on T, B cells, and antibody response in vivo.