T22 ([Tyr5,12, Lys7]-polyphemusin II) has been shown to have strong anti-human immunodeficiency virus (HIV) activity. The precise mechanism of action of T22 on HIV-replication has not been elucidated yet, nor have the targets of T22 been identified. However, our previous research suggested that T22 exerts its effect by blocking virus-cell fusion and that T22 might interact with an HIV envelope protein and/or a T-cell surface protein. Herein we use a novel biosensor based on the principles of surface plasmon resonance (BIAcore) to demonstrate that T22 binds specifically to both gp120 (an envelope protein of HIV) and CD4 (a T-cell surface protein) and that both bindings can be inhibited by an anti-T22 antibody. The data obtained suggest that T22 inhibits virus-cell fusion through the double binding to the above two proteins.