In addition to its activities as a growth factor, recent studies suggest an immunoregulatory role for transforming growth factor-beta (TGF-beta). In this context we have demonstrated that TGF-beta is a potent chemotactic factor in vitro for human T lymphocytes at a concentration of 40 fM and for monocytes at a concentration of 0.4 fM but that it has no chemotactic activity for neutrophils. Furthermore, using an assay of lymphocyte subset chemotaxis we have been able to show that TGF-beta can induce migration of both CD4+ and CD8+ T lymphocytes in vitro. This study provides further evidence that TGF-beta acts as a cytokine, being able to attract T lymphocytes and monocytes to sites of inflammation. Its role in the pathogenesis of inflammatory reactions is likely to be complex.