A hybridoma producing the monoclonal antibody HIS45 was isolated from a fusion between spleen cells from a Balb/c mouse immunized with rat bone marrow cells and the fusion partner SP2/0. This antibody recognizes a determinant present on the majority of peripheral T and B cells and a small percentage of thymocytes. In a xenogeneic mixed leucocyte reaction HIS45 completely inhibits the proliferation of responding cells. HIS45 does not inhibit natural killer cell-mediated lysis. Comparison with other antibodies which have been reported to effect lymphocyte function fail to reveal any which have similar properties.