Human IL-10 (hIL-10) is a newly described cytokine that was originally identified as a cytokine synthesis inhibitory factor regulating the production of several pro-inflammatory cytokines such as IL-1 alpha, IL-1 beta, IL-6, IL-8, TNF-alpha, and IFN-gamma. Additionally, hIL-10 also inhibits the macrophage-dependent proliferative response of CD4+ T lymphocytes to Ag stimulation, and it down-regulates the constitutive class II MHC expression on human monocytes. We report hIL-10 to be a potent and specific chemotaxin for human T lymphocytes with optimum activity in the range between 10 and 100 U/ml. Checkerboard analysis shows the activity to be chemotactic and not chemokinetic. The chemotactic activity is directed toward CD8+ T lymphocytes and not towards CD(4+)-enriched cells. Also, hIL-10 lacks chemotactic activity toward human monocytes or neutrophil granulocytes. Further, we found that hIL-10 inhibits the chemotactic response of CD4+, but not CD8+, T cells toward IL-8. Because hIL-10 can be produced by several cells including CD4+ T cells of the Th2 type, our results suggest that hIL-10 participates in a complex regulatory circuit between CD4+ and CD8+ T cells with implications for the control of lymphocyte-mediated inflammatory responses.