In prior studies, we showed that most CD8+ cells infiltrating skin lesions of CD3+CD4+ mycosis fungoides were CD3+ T-lineage tumor-infiltrating lymphocytes (TIL) whose overall phenotype was suggestive of MHC-restricted cytotoxic T lymphocytes (CTL). However, their lack of cytotoxic-associated granzyme A mRNA suggested that they might be unactivated CTL precursors. In this study, we used single- and double-label immunohistologic techniques to assess the expression of TIA-1-reactive protein and HLA-DR by these CD8+TIL. Monoclonal antibody TIA-1 recognizes a novel family of proteins expressed preferentially by cytotoxic cells, including some that lack granzyme A. HLA-DR is a marker of T-cell activation. Single-label studies of 32 cases showed that CD8+TIL and TIA-1+ cells constituted a variable minority of the total cellular infiltrate and had a similar distribution. Double-label studies of 14 cases showed that in most instances the aggregate phenotype of the majority of CD8+TIL was CD3+TIA-1+HLA-DR+CD56-CD57-. These findings suggest that many of the CD8+TIL within skin lesions of CD3+CD4+ mycosis fungoides are activated, MHC-restricted CTL.