Pepsin-solubilized collagen VI in triple-helical and heat-denatured, unfolded form was shown to promote Mg(2+)- and Mn(2+)-dependent attachment and spreading of various cell lines. On the triple-helical substrate no inhibition of cell adhesion was observed with several synthetic RGD peptides except in the case of A375 melanoma cells. In contrast, adhesion to the unfolded substrate was highly sensitive to RGD inhibition. Nine synthetic peptides were designed according to 10 RGD sequences present in the triple-helical sequence of human collagen alpha 1(VI), alpha 2(VI), and alpha 3(VI) chains. Only one peptide, corresponding to the C-terminal end of alpha 3(VI) chain, showed substantial inhibitory activity, whereas several peptides were active in direct adhesion assays when used as albumin conjugates. Inhibition tests with antibodies to integrin subunits, affinity chromatography, and ligand binding with purified integrins (alpha 1 beta 1, alpha 2 beta 1, alpha V beta 3, and alpha IIb beta 3) were used to identify collagen VI receptors. Binding to the triple-helical substrate is mediated by alpha 1 beta 1 and alpha 2 beta 1 integrins. Binding of both integrins to collagen VI was weaker than that to collagens I and/or IV. Recognition of the denatured substrate is mediated by beta 1 and beta 3 integrins. Activity was shown for alpha 5 beta 1 and alpha V beta 3 and weakly for alpha IIb beta 3 but not all alpha subunits possibly involved were identified. Distinct sets of receptors were also involved in A375 cell binding to triple-helical (beta 1-mediated) and denatured (beta 3-mediated) collagen VI, even though in this case both interactions could be efficiently inhibited by RGD peptides.