Vascular smooth muscle cells perform the important function of modulation of vascular extracellular matrix. Because integrins mediate many cell-matrix interactions, the role of integrins in reorganization of collagen by cultured human vascular smooth muscle cells was studied. Immunoprecipitation demonstrated that human vascular smooth muscle cells express multiple β₁ integrins. Monoclonal antibody A2-IIE10 (a blocking anti-α² antibody) inhibited adhesion of smooth muscle cells to collagen by 31%. The blocking anti-α¹ antibody 1B3.1 inhibited adhesion by 40%, whereas a blocking anti-α³ antibody had no effect on adhesion. When 1B3.1 and A2-IIE10 were both used, a 79% reduction in adhesion was observed, indicating that active α¹ and α² integrins cooperatively mediate adhesion. The blocking anti-β₁ antibody Mab13 abolished smooth muscle cell–mediated gel contraction, and the α²-blocking antibody A2-IIE10 had a dose-dependent partial inhibitory effect (37%). In contrast, blocking antibodies to α¹ and α³ had no effect. When anti-α¹ (1B3.1) and anti-α² (A2-IIE10) monoclonal antibodies were combined, no synergistic effect on inhibition of gel contraction was observed. Surprisingly, collagen gel contraction was inhibited by 46% by an anti-β₁ antibody (TS2/16) known for its stimulatory effect on cell adhesion. Thus, whereas α¹β₁ and α²β₁ integrins both participate in adhesion of vascular smooth muscle cells to collagen, only α²β₁ integrins mediate collagen reorganization. In addition, collagen reorganization appears to be a dynamic process, adversely affected by excessive adhesion strengthening.