The lymphocytes from patients with Graves' disease or from healthy subjects have been cultured in vitro either alone or with phytohemagglutinin (PHA). After six days the culture supernatants have been assayed for their human thyroid-stimulating activity by measuring increases in adenosine 3',5' monophosphate (cyclic AMP) in human thyroid slices with which the supernatants have been incubated. Significant levels of human thyroid stimulator activity were found in the culture in which Graves' lymphocytes were cultured with PHA. This activity has been abolished by precipitation of the IgG from the culture supernatant with goat antihuman IgG serum. In contrast, when Graves' lymphocytes were cultured alone, or when control lymphocytes were cultured either alone or with PHA, there was no overall significant production of human thyroid-stimulating immunoglobulin (HTSI). It is concluded that Graves' lymphocytes can be stimulated by PHA to produce HTSI in vitro. Since PHA is known to stimulate only the T lymphocytes, which do not themselves elaborate immunoglobulins as the B lymphocytes do, the above observations indicate a cooperation between T and B lymphocytes in the production of HTSI, at least in this system.