Human T cell hybrids were generated by fusing lectin-activated normal and leukemic human T cells with an aminopterin-sensitive human T cell line. This mutant cell line, designated CEM-T15, was derived from the human T cell line CEM after chemical mutagenesis with ethane methylsulfonate and subsequent culture in medium containing 6-thioguanine. After polyethylene glycol-induced fusion, the cells were cultured in hypoxanthine-aminopterin-thymidine selective medium. More than 5 wk after fusion, evidence for successful hybridization was obtained by three independent criteria: (a) The majority of the cultures contained cells expressing the OKT3 surface antigen: this antigen is expressed on normal T cells but not on CEM-T15 cells. (b) Most of the cultures contained polyploid cells. (c) Some of the cultures provided helper activity in the generation of antibody-forming cells. This functional activity is absent from the CEM-T15 parental cell line. Evidence for functional stability of the hybrids greater than 20 wk after fusion was provided by several clones that not only continue growing exponentially but also maintain expression of OKT3 surface antigen and high levels of helper function. These T cell hybrids constructed using antigen-specific human T cells should be of considerable importance in further studies of the immunobiology of human T cells.

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A monoclonal antibody, PVR-1, was obtained after hybridization of X63Ag8.653 murine myeloma cells with spleen cells from a mouse immunized with human lymphocytes. It recognizes a 175,000-to 185,000-dalton surface antigen present on -"80% ofnormal human peripheral T lymphocytes, 50% ofnon-T non-B cells, and < 10% of B cells as determined by complementdependent microcytotoxicity. It is also present on various leukemia T cells, on some but not all T lymphoblastoid cell lines, and on a small fraction of some B lymphoblastoid cell lines. Some B-cell chronic lymphocytic leukemia cells also express the PVR-11 antigen. Functional analysis of normal human T lymphocytes demonstrated that the PVR-11-depleted T-cell subset contains the precursors ofboth cytotoxic and suppressor cells but lacks helper cells. On the other hand, cytotoxic effector T cells express the antigen. These results demonstrate that antigenic determinants with relatively wide tissue distribution can dissect functionally distinct human immunoregulatory T-cell subsets.During the last several years, a number of antibodies that recognize distinct surface membrane antigens on human T lymphocytes have been described. These antibodies have different specificities that can be grouped into at least three distinct categories. The first group includes antibodies reacting with all or