A novel murine monoclonal antibody (MAb) 1D8 was produced after immunization with the nylon wool-adherent fraction of human peripheral blood mononuclear cells (PBMNC). Its reactivity pattern was studied on a panel of hemopoetic normal cells, cell lines and malignancies. Mab 1D8 detects a lymphocyte-specific surface antigen expressed on a major subpopulation of mature B lymphocytes as well as on a minor T-cell subset. Activation-related changes in the expression of 1D8 molecule were observed on both B and T lymphocytes. As compared with the pattern of known activation-associated antigens, 1D8 seems to play a role in the early stages of lymphocyte activation. The antigen could not be immunoprecipitated by the conventional methods for glycoproteins.