Leukocyte infiltration is characteristic of lung injury and fibrosis, and its role during tissue repair and fibrosis is incompletely understood. We found that overexpression of IL-5 in transgenic mice (IL-5(TG)) or by adenoviral gene transfer increased bleomycin (blm)-induced lung injury, fibrosis, and eosinophilia. Surprisingly, blm-treated IL-5-deficient (IL-5(-/-)) mice also developed pronounced pulmonary fibrosis but characterized by marked T lymphocyte infiltration and absence of eosinophilia. In both murine strains however, induction of lung TGF-beta expression was evident. Purified lung eosinophils from blm-treated IL-5(TG) mice stimulated alpha-smooth muscle actin and collagen expression in mouse lung fibroblasts, without affecting proliferation. Furthermore instillation of purified eosinophils into murine lungs resulted in extension of blm-induced lung fibrosis, thus confirming a role for eosinophils. However, lung T lymphocytes from blm-treated IL-5(-/-) mice were able to stimulate fibroblast proliferation but not alpha-smooth muscle actin or collagen expression. Blocking T cell influx by anti-CD3 Abs abrogated lung fibrosis, thus also implicating T lymphocytes as a key participant in fibrosis. Pulmonary fibrosis in IL-5(TG) mice was preferentially associated with type 2 cytokines (IL-4 and IL-13), whereas fibrotic lesions in IL-5(-/-) animals were accompanied by proinflammatory cytokine (TNF-alpha, IL-1beta, and IFN-gamma) expression. We suggest that eosinophils and T cells contribute distinctly to the development of blm-induced lung fibrosis potentially via their production of different cytokine components, which ultimately induce TGF-beta expression that is intimately involved with the fibrosis.