An object of the present invention is to provide a technique to distinguish between T.sub.reg cells and activated T cells in a live state. Another object of the present invention is to provide a pharmaceutical composition for immunostimulation that can reduce the number of T.sub.reg cells in vivo and effectively express the immune response of activated T cells. In the method of the present invention, T.sub.reg cells are detected from test cells containing (i) regulatory T cells and (ii) at least one type of cell selected from the group consisting of naive T cells and activated T cells, wherein expressions of folate receptor 4 on the surfaces of cells are measured and T.sub.reg cells are detected using the expressions as an indicator. The present invention uses anti-folate receptor 4 antibody or folate receptor 4-binding fragment as an active ingredient contained in a pharmaceutical composition for immunostimulation.