CD8⁺ T cells in different activation states have been difficult to identify phenotypically. In this study we have investigated whether Mac-1 (CD11b) expression can be used as a criterion to distinguish between recently activated effector cells and memory cells belonging to the CD8⁺ T cell subset. Polyclonal virus-specific effector and memory CD8⁺ T cells from lymphocytic choriomeningitis- and vesicular stomatitis virus-infected mice were visualized through staining for intracellular IFN-γ or binding of MHC–peptide tetramers, and Mac-1 expression was evaluated. Naive T cells and most virus-specific memory CD8⁺ T cells express little or no Mac-1 independent of the virus model employed. In contrast, the majority of CD8⁺ T cells present during acute infection express a significant level of Mac-1 and, similarly, Mac-1 expression is found on secondary effectors generated in response to viral re-exposure. We therefore suggest that high Mac-1 expression defines a subset of circulating effector cells and that the presence of this marker on antigen-specific CD8⁺ T cells signifies recent activation.