We reported previously that thyrotropin (TSH) enhanced the in vitro antibody response to a T cell-dependent antigen, sheep red blood cells (SRBC), as determined by direct plaque-forming cell (PFC) assay. The present studies were designed to determine the possible immunoregulatory function of TSH on lymphocytes immunized with the T-independent antigen Brucella abortus-TNP (BA-TNP) and the cellular components involved in such function. We report here that TSH enhanced the in vitro antibody response to BA-TNP as determined by direct PFC assays. Cell depletion studies showed that the TSH effect, although independent of macrophages, required the presence of T cells. Thus, pituitary - and possibly leukocyte - TSH appears to function as a lymphokine which may act via T cells to augment antibody production.