Lymphoid cells obtained from a mediastinal mass and bone marrow as well as peripheral blood of a patient with poorly differentiated diffuse malignant lymphoma were found to have both E and complement receptors. Addition of the bone marrow lymphoblasts to normal human T-lymphocytes induced a suppression of normal T functional activity as measured by a local xenogeneic graft versus host reaction. Incubation of the same cells with thymic humoral factor (THF), a thymic hormone, reversed their functional activity from suppression to help. Control studies with normal T-suppressor lymphocytes also showed a reversal of function to helper activity upon incubation with THF. In studies of normal T-lymphocytes with monoclonal antibodies, incubation with THF induced no change in phenotype in either theophylline-sensitive (suppressor) cells or theophylline-resistant (helper) cells. The ability of THF to modulate helper and suppressor activities suggests that this or similar agents may provide a useful means of therapy for disorders of immunoregulation.