Microvascular endothelial cells (RFCs) cultured in twodimensional (2D) cultures proliferate rapidly and exhibit an undifferentiated phenotype. Addition of transforming growth factor ␤ 1 (TGF ␤ 1) increases fibronectin expression and inhibits proliferation. RFCs cultured in three-dimensional (3D) type I collagen gels proliferate slowly and are refractory to the anti-proliferative effects of TGF ␤ 1. TGF ␤ 1 promotes tube formation in 3D cultures. TGF ␤ 1 increases fibronectin expression and urokinase plasminogen activator (uPA) activity and plasminogen activator inhibitor-1 (PAI-1) levels in 3D cultures. Since the TGF ␤ type I and II receptors have been reported to regulate different activities induced by TGF ␤ 1, we compared the TGF ␤ receptor profiles on cells in 2D and 3D cultures. RFCs in 3D cultures exhibited a significant loss of cell surface type II receptor compared with cells in 2D cultures. The inhibitory effect of TGF ␤ 1 on proliferation is suppressed in transfected 2D cultures expressing a truncated form of the type II receptor, while its stimulatory effect on fibronectin production is reduced in both 2D and 3D transfected cultures expressing a truncated form of the type I receptor. These data suggest that the type II receptor mediates the antiproliferative effect of TGF ␤ 1 while the type I receptor mediates the matrix response of RFCs to TGF ␤ 1 and demonstrate that changes in the matrix environment can modulate the surface expression of TGF ␤ receptors, altering the responsiveness of RFCs