Interleukin-13 (IL-13) is a recently identified cytokine that is secreted by activated T cells and regulates inflammatory responses. We have investigated the effects of IL-13 on isolated human osteoblast-like cells (hOB). IL-13 dose-dependently (1-100 pmol/L) reduced the incorporation rate of [3H]thymidine in hOB cells by more than 50%. Using a cell metabolic assay as well as direct cell counting, we found that treatment with IL-13 lead to a decrease in hOB cell number. The effect was both time and dose dependent, and after 12 days of culture, treatment with IL-13 (0.1 nmol/L) caused a 70% decrease in the number of cells. Also, IL-13 increased the levels of IL-6 messenger ribonucleic acid in hOBs, as measured by ribonuclease protection assay, and stimulated secretion of IL-6 into culture supernatants. In conclusion, IL-13 inhibits cell proliferation and increases IL-6 formation in human osteoblasts. Our findings suggest that IL-13 may cause bone loss due to impaired osteoblastic growth and IL-6-induced osteoclast recruitment.