ICOSL, a newly identified member of the B7 superfamily, plays a major role in immune responses. In this study, a functional anti-human ICOSL monoclonal antibody (MAb) 3B3 was obtained and characterized by means of flow cytometry, Western blot, and competition assay. This MAb could specifically recognize a distinct epitope of the ICOSL molecule. As a functional antibody, MAb 3B3 could inhibit the proliferation of T lymphocytes stimulated by ICOSL-L929 transfectants. Furthermore, it could enhance IgG production of PWM-driven B cells. The results indicate that the ICOS-ICOSL signal is critically involved in specific humoral immunity.