Human T cells expressing CD56 are capable of tumour cell lysis following activation with interleukin-2 but their role in viral immunity has been less well studied. Proportions of CD56+ T cells were found to be highly significantly increased in cytomegalovirus-seropositive (CMV+) compared with seronegative (CMV−) healthy subjects (9·1 ± 1·5% versus 3·7 ± 1·0%;P< 0·0001). Proportions of CD56+ T cells expressing CD28, CD62L, CD127, CD161 and CCR7 were significantly lower in CMV+ than CMV− subjects but those expressing CD4, CD8, CD45RO, CD57, CD58, CD94 and NKG2C were significantly increased (P< 0·05), some having the phenotype of T effector memory cells. Levels of pro-inflammatory cytokines and CD107a were significantly higher in CD56+ T cells from CMV+ than CMV− subjects following stimulation with CMV antigens. This also resulted in higher levels of proliferation in CD56+ T cells from CMV+ than CMV− subjects. Using Class I HLA pentamers, it was found that CD56+ T cells from CMV+ subjects contained similar proportions of antigen-specific CD8+ T cells to CD56− T cells in donors of several different HLA types. These differences may reflect the expansion and enhanced functional activity of CMV-specific CD56+ memory T cells. In view of the link between CD56 expression and T-cell cytotoxic function, this strongly implicates CD56+ T cells as being an important component of the cytotoxic T-cell response to CMV in healthy carriers.