AbstractT lymphocytes express a T cell antigen receptor (TcR) complex composed of either an TcR α/β or TcR γ/δ heterodimer in noncovalent association with the CD3 glycoproteins. CD28, a 44‐kDa disulfide‐linked homodimer, is present on the surface of the majority of TcR α/β‐bearing T lymphocytes. Monoclonal antibodies (mAb) directed against CD28 potentiate activation signals delivered through the CD3/TcR α/β complex. Herein, we demonstrate that CD28 is expressed on ∼40%–60% of TcR γ/δ‐bearing T lymphocytes in most donors. Anti‐CD28 mAb substantially augmented proliferative signals delivered through the TcR γ/δ, demonstrating the presence of functional CD28 molecules on TcR γ/δ‐bearing T lymphocytes. The majority of TcR γ/δ thymocytes also expressed CD28.