We report a new inhibitory activity of CD8+CD57+ peripheral blood lymphocytes from allo-bone marrow transplant patients. Positively selected CD8+CD57+ lymphocytes act as potent inhibitors of allospecific cytolytic T lymphocyte and lymphokine activated killer cell cytolytic activities. These suppressor cells are mature T cells expressing the CD2, 5, 7, CD3-TcR alpha beta complex, and lack natural killer cell markers as well as cytolytic function. Their inhibitory activity on both cytolytic processes and T-cell proliferation is mediated by a non- antigen-specific soluble factor released by CD8+CD57+ cells in culture supernatants. Preliminary characterization suggests that the CD8+CD57+ cells' inhibitory activity is mediated by a low molecular weight, glycosylated factor as indicated by its less than 1S coefficient of sedimentation and its binding to concanavalin A lectin.