Several reports support the view that growth hormone (GH) promotes proliferation and cytotoxicity by T cells in a mixed leukocyte culture (MLC). The present study was undertaken to begin to determine the mechanism of action of GH on the MLC in vitro. First, we determined that peripheral blood mononuclear cells (PBMC) cultured with mitomycin-treated allogeneic PBMC in an MLC in the presence of exogenously added rhGH develop an augmented proliferative (25–100%) and cytotoxic response (50–600%). We next examined the possibility that GH may promote alloresponses by inducingγ-interferon (IFNγ) production. In these experiments, in situ hybridization was used to determine the frequency of cells expressing mRNA for IFNγ. It was observed that GH increased significantly the frequency of cells expressing mRNA for IFNγ(100-800%). To determine the site of action of rhGH, we evaluated the response of purified T cells to alloantigens in the presence of rhGH. The addition of rhGH to an MLC had no demonstrable effect when purified T cells were used as the responding population. However, when T cells were reconstituted with autologous mitomycin-treated PBMC and used as the responding population, rhGH augmented proliferation and cytotoxicity. Taken together, these data show that rhGH augments proliferation, cytotoxicity and IFNγproduction during an MLC, and at least part of the action of rhGH appears to be on the autologous antigen-presenting cell. © 1997 S. Karger AG, Basel.