The effects of interleukin-11(IL-11) on the differentiation of osteoblast precursors was tested using a bone nodule forming assay in rat calvaria cell cultures. IL-11 caused a dose dependent inhibition of nodule formation, with 500 U/ml IL-11 resulting in complete inhibition of nodule formation. IL-11 also caused a reduction in alkaline phosphatase expression in these cultures. These effects are similar to, but more potent than, the actions of IL-6 on these cells. These results indicate that IL-11 is an osteotropic cytokine and suggest that IL-11 may be an important inhibitor of bone formation in health and disease.