IL-7 has previously been shown to stimulate proliferation of immature CD4-CD8- thymocytes from fetal and adult mice. We show that IL-7 mRNA levels are much higher in the day-14 fetal thymus than in the adult thymus, i.e., when the thymus is primarily composed of CD4-CD8- cells. Inasmuch as IL-7 may, therefore, be one of the first cytokines to which pre-T cells from the fetal liver are exposed when they migrate to the thymus during development, we hypothesized that IL-7 may promote differentiation of pre-T cells from the fetal liver. We found that human rIL-7 promotes growth of cells obtained from the liver of fetal mice at day 14 of gestation. Moreover, we found that IL-7 stimulated expression of mRNA encoding TCR-gamma, alpha-, and beta-genes. The size of the transcripts induced by IL-7 suggests that TCR-gamma was expressed in a rearranged form in response to IL-7, whereas TCR-alpha and -beta genes appeared to be expressed from nonrearranged loci. Culture of fetal liver cells for 7 days with IL-7 also caused the appearance of a population of cells expressing Thy-1 and Pgp-1 Ag, a phenotype similar to that seen in the early (day-14) fetal thymus. The Thy-1+ Pgp-1+ population was enriched in TCR-gamma mRNA, as determined by flow cytometric sorting followed by Northern blot analysis. These data support the hypothesis that IL-7 can stimulate some of the early events associated with thymocyte differentiation in the fetus.