Oncostatin M (OSM) is a member of the interleukin 6 (IL-6) family of cytokines and was originally identified by its ability to inhibit proliferation of melanoma cells but augment the growth of normal fibroblasts. OSM has pleiotropic effects on many different cell types, but here we focus on its ability to inhibit the proliferation of cell lines derived from several tumour types, including breast carcinoma, ovarian cancer, melanoma, glioma and lung carcinoma. The inhibition of proliferation of several cancer cell lines by OSM is associated with alterations in cellular morphology and with phenotypic changes that are consistent with the induction of differentiation of these cells. These observations raise the possibility that OSM could have therapeutic potential.