Anti delta IK17 monoclonal antibody was produced by fusing SP2/0/Ag 14 myeloma with spleen cells of BALB/c mice immunized with normal human thymocytes. a delta IK17 antibody recognizes a 44kD cell surface protein detected on human lymphocytes. delta IK17 is expressed on human thymocytes, CD4+ and CD8+ T cell subsets, B, NK cells, as well as on activated cells. The antigen is detected on cells during the early, intermediate and late stages of lymphocyte maturation. In addition, the expression of the antigen is correlated with ontogenesis. A T+ delta IK17+ subpopulation responded poorly to TPA stimulation and provided a better helper signal for PWM-induced IgM synthesis than T+ delta IK17- cells. In addition, different levels of delta IK17 expression were detected in several hematological diseases tested.