The co‐stimulatory B7 molecules (B7‐1 and B7‐2) are expressed on professional antigen‐presenting cells in mice. In this study, we demonstrate that B7‐1 (CD80) and B7‐2 (CD86) are also expressed on murine T cells in the absence of major histocompatibility complex class II molecules. The temporal expression of these two molecules on T cells varies with the state of activation where resting T cells express B7‐2 but show little or no expression of B7‐1. Following activation, B7‐2 expression is down‐regulated and there is a concomitant increase in the expression of B7‐1 on the cell surface which peaks at about 72 h. Thus these two co‐stimulatory molecules are reciprocally expressed on the T cell surface. This pattern of expression of B7‐1 and B7‐2 on T cells suggests that these two molecules may have different roles in the generation and regulation of immune responses.