The thymus provides the microenvironment that is optimal for T-cell differentiation. The most immature cells in the human thymus express the stem-cell marker CD34 and they respond to cytokines, including stem-cell factor (SCF) and interleukin-7 (IL-7). For the normal progression of T-cell development these two cytokines appear to be vital. We have established and characterized a human pre-T-cell line, PER-487, which mirrors this requirement. This study shows that the simultaneous presence of IL-7 and SCF produces a proliferative response far exceeding additive effects. Furthermore, providing these signals in succession did not achieve the effect observed when they were provided simultaneously. This finding suggests that the effect was not mediated via secretion of molecules or modulation of surface expression. The convergence of the signal transduction pathways of the two cytokines is not known, thus cell line PER-487 provides a unique model for studying the synergistic interaction of IL-7 and SCF.