Antigen-specific T-ceil activation is initiated through the T-cell receptor. Recent evidence has shown that a number of additional T-cell surface receptors serve to regulate the responses of antigen-activated T cells. One such molecule, CD28, is a member of a heterophilic cell adhesion complex, and is the receptor for the B-cell-restricted B7/BB-1 antigen. As Carl June, Jeffrey Ledbetter, Peter Linsley and Craig rhompson review here, CD28 serves as the surface component of a novel signal transduction pathway that modulates T-cell lymphokine production and increases the resistance of T-cell responses to various immunosuppressive agents. interaction between the T cell and antigen that is expressed in conjunction with either MHC class I or class II molecules on an antigen-presenting celi initiates a cascade of biochemical events that collectively are termed antigen-specific T-cell activation. However, while the en-