These studies investigate the effects of exogenously administered recombinant human IL-7 (rhIL-7) on mouse leukocyte subsets in vivo in normal and tumor-bearing mice. The administration of rhIL-7 to normal mice caused a pronounced leukocytosis (three- to fivefold increase over background) in the spleen and lymph nodes, with B-lineage and T cells, NK cells, and macrophages all being increased. CD8+ T cells increased disproportionately, such that the CD4 to CD8 ratio decreased dramatically. The rhIL-7-induced effects were dose-dependent, increased with duration of treatment, and were reversible after cessation of rhIL-7 administration. T cell number increases after rhIL-7 treatment were primarily a result of an expansion of the peripheral T cell population. Importantly, splenocytes from rhIL-7-treated mice have enhanced proliferative responses to various T cell stimuli in vitro and were able to potentiate an allogeneic CTL response in vivo. The rhIL-7-induced changes in T cell number and the CD4 to CD8 ratio also were observed in mice bearing early Renca renal adenocarcinoma pulmonary metastases, and these changes coincided with up to a 75% reduction in pulmonary metastases. Overall, these results demonstrate that the administration of rhIL-7 to mice profoundly increases the number of B and T cells, and reduces the number of pulmonary metastases. The results also suggest that IL-7 may be useful for restoring lymphoid subsets in immunosuppressed hosts and in enhancing T cell-mediated immune responses. Such effects may be useful in the treatment of microbial diseases and cancer.