The effect of IL-7 on the growth of thymic T lymphocytes was investigated by adding recombinant IL-7 into cell suspension cultures and submersion organ cultures (SOC) of murine fetal thymuses (FT) and newborn thymuses (NBT). FT and NBT were obtained from C57BL/6 mice at day 15 of gestational age and at day 3 after birth, respectively. In both cell suspension cultures and SOC, addition of IL-7 highly improved the cell recovery. In cell suspension cultures, addition of IL-7 resulted in the growth of gamma delta T-cells from FT-cells, whereas the same cytokine promoted the growth of both alpha beta and gamma delta T-cells from NBT-cells. These results may indicate that this cytokine is able to support the proliferation of T-cells of both alpha beta and gamma delta lineages. In marked contrast, in SOC, addition of IL-7 resulted in the growth of gamma delta T-cells not only in FT but also in NBT, despite the fact that the SOC of NBT without exogenous cytokine exclusively promoted the growth of alpha beta T-cells. A similar effect was also seen when IL-2 was added to NBT-SOC, though the skewing to gamma delta lineage was not so strong as in the case of IL-7. In addition, we found that IL-7 mRNA is expressed in the day 15 FT at a much higher level than in the adult thymus. These results strongly suggested that the production of a large amount of IL-7 synthesized in the FT is one of the major factors leading to the generation of gamma delta T-cells in FT.