Collagen, either alone or in combination with other materials, is an important natural biomaterial that is used in a variety of tissue-engineering applications. Cell adhesion and migration of cells within collagen-based biomaterials may be controlled by modifying the adhesive properties of collagen. Furthermore, spatially controlling the adhesiveness of the collagen may allow controlled localization or redistribution of cells. A method is presented for covalently coupling peptides that contain the well-characterized arginine-glycine-aspartic acid adhesion sequence directly to type I collagen monomers prior to fibrillogenesis. A heterobifunctional coupling agent was used to create stable amide and disulfide bonds with the lysine residues of the collagen monomers and the cysteine termini of the peptide molecules, respectively. The degree of conjugation could be controlled by changing the reaction conditions (ratios of reactants added and the length of incubation). The microstructure and gelation times of gels composed of covalently modified collagen were similar to those of unmodified gels. Cell adhesion on adsorbed monolayers of modified collagen was quantified using a well-established clonal cell line (K1735 murine melanoma). Cell adhesion was found to increase with both increasing degree of conjugation and increasing ratio of modified to unmodified collagen.