Objective - To identify extracellular proteoglycans produced by canine melanoma cell lines and analyze the effect of transforming growth factor-β1 (TGF-β1), insulin-like growth factor-I (IGF-I), and hepatocyte growth factor (HGF) on these proteoglycans. Sample Population - 3 canine melanoma cell lines lie, CML-1, CML-6M, and CML-10c2). Procedure - Extracellular proteoglycans were analyzed by use of metabolic labeling and western immunoblot analysis. The effect of TGF-β1 on cell proliferation was determined by incorporation of 5-bromo-2′-deoxyuridine. Results - The CML-1 and CML-6M melanoma cell lines produced 2 main extracellular proteoglycans. One of them was identified as versican, a proteoglycan found in undifferentiated human melanoma cell lines. The CML-10c2 cells produced a small amount of extracellular proteoglycans. Addition of TGFP-β1 (1.25 to 6.25 ng/ml) increased the release of sulfated proteoglycans into the medium. The TGF-β1 had mainly a posttranslational effect, because it increased the molecular mass of the sulfated bands. Addition of IGF-I (50 ng/ml) slightly increased production of proteoglycans in the CML-6M cell line, whereas HGF (50 ng/ml) did not have any effect on proteoglycan production. Conclusions and Clinical Relevance - The proteoglycan content and response to TGF-β1 treatment for CML-1 and CML-6M canine melanoma cell lines are similar to that for undifferentiated human melanoma cell lines. In contrast, CML-10c2 cells produced a low amount of proteoglycans with high molecular weight. Because these extracellular proteoglycans are involved in the control of cell adhesion, proliferation, and migration, they may play an important role in the progression of melanomas in dogs.