Type 1 interferons (IFN-α/β) have recently been shown to inhibit interleukin-7 (IL-7)-induced growth and survival of early B-lineage cells. The CD3- CD4- CD8- (triple negative; TN) thymocytes from normal mice strongly proliferated upon stimulation with IL-7 in suspension culture. Such an IL-7-induced proliferation was suppressed by the addition of IFN-α/β, but a fraction of the TN thymocytes still showed proliferation. The IL7-induced growth of TN thymocytes from scid mice, which lack the CD44- CD25- subpopulation, was completely inhibited by the addition of IFN-α/β. The IL-7 induced proliferation of CD4- CD8- thymocytes from T-cell receptor (TCR) transgenic mice, the majority of which are CD3+ CD44- CD25-, was resistant to IFN-α/β-mediated suppression. In fetal thymus organ cultures (FTOC), the addition of IL-7 greatly increased the population of CD4- CD8- CD44+ CD25+ thymocytes and IFN-α/β inhibited this IL-7-driven expansion. In contrast, the addition of IL-7 markedly decreased the percentages of CD4- CD8- CD3- CD44- CD25- cells, and IFN-α/β reversed the effect and increased the subpopulations of CD44- CD25+ and CD44- CD25-. Finally, IFN-β mRNA was found to be expressed in the thymus. The data suggest that type 1 interferons inhibit IL-7-driven proliferation of TN thymocytes, but do not block the normal differentiation process.