Prostatic carcinoma cells have a propensity to metastasize to bone, and we propose that this phenomenon may be promoted by the adhesion of metastatic cells to bone matrix. Bone matrix is produced by osteoblasts, and we have developed an in vitro model of bone matrix by isolating the substratum deposited by human osteoblast-like U2OS cells. The collagenous nature of this matrix was demonstrated by the incorporation of [3H]proline and its subsequent release by purified collagenase. Both U2OS matrix and purified type I collagen stimulated the adhesion of human PC-3 prostatic carcinoma cells. Human laminin supported adhesion to a much lesser extent, and PC-3 cells did not adhere to fibronectin. Adhesion of PC-3 cells to U2OS matrix closely resembled adhesion to purified type I collagen with respect to (a) inhibition by a collagen-derived peptide and by antibodies raised against alpha 2 or beta 1 integrin collagen receptor subunits; (b) lack of inhibition by RGD (Arg-Gly-Asp) peptides; (c) stimulation by Mn2+ and Mg2+ ions but not by Ca2+ ion; and (d) stimulation by the phorbol ester PMA (phorbol 12-myristate 13-acetate). This adhesion was also stimulated (2.3-fold) by transforming growth factor beta (TGF-beta), which is a major bone-derived growth factor. We conclude that human osteoblast-like matrix is an adhesive substrate for PC-3 prostate carcinoma cells. This adhesion appears to be mediated by the interaction of alpha 2 beta 1 integrin on PC-3 cells with matrix-derived collagen. The stimulation of this adhesion by TGF-beta suggests that the co-expression of TGF-beta and type I collagen in bone may synergistically facilitate the adhesion of metastatic cells to bone matrix proteins and thereby increase their localization in the skeleton.