Purified human interleukin 6 (IL 6) was found to stimulate the proliferation of human tonsillar and peripheral rosetting T cells subliminally activated with phytohemagglutinin (PHA). This response seemed independent of IL2 but highly dependent on the presence of accessory cells. Indeed, when accessory cell-depleted tonsillar T cells were activated with PHA and exposed to IL6, only minimal proliferations were observed. A similar result was obtained with IL1. However, a combination of these two cytokines induced strong proliferations, indicating that IL 1 and IL 6 play a synergistic role in the interactions between accessory cells and T lymphocytes.