Purpose. Production of active transforming growth factor-β (TGF-β) by human osteosarcoma may contribute to malignant progression through mechanisms that include induction of angiogenesis, immune suppression and autocrine growth stimulation of tumor cell growth. To study events associated with induction of cell proliferation by TGF-β, we have evaluated the TGF-β pathway in two murine osteosarcoma cell lines, K7 and K12. Results. Northern and immunohistochemical analyses show that each cell line expresses TGF-β1 and TGF-β3 mRNA and protein. Both cell lines secrete active TGF-β1 and display a 30-50% reduction in growth when Cultured in the presence of a TGF-β blocking antibody. Expression of TGF-β receptors TSSRI, TSSRII and TSSRIII is demonstrated by affinity labeling with 125I-TGF-β1, and the intermediates, Smads 2, 3 and 4, are uniformly expressed. Smads 2 and 3 are phosphorylated in response to TGF-β, while pRb phosphorylation in each osteosarcoma cell line is not affected by either exogenous TGF-β or TGF-β antibody. Conclusions. The data implicate events downstream of Smad activation, including impaired regulation of pRb, in the lack of a growth inhibitory response to TGF-β, and indicate that this murine model of osteosarcoma is valid for investigating the roles of autocrine TGF-β in vivo.