We inoculated the KLE human endometrial cancer, MCF-7 and ZR-75 human breast cancer, and PC-3 human prostate cancer cells into three-dimensional type I collagen gel system that contained uniformy dispersed MG-63 osteoblast-like cells. Then, we analyzed the morphological evidence of osteoblasts reaction, local invasion around the inoculated cancer cells and expression of the cathepsin D and urokinase-type plasminogen activator (uPA) around the sites of inoculation using immunocytochemistry. The prostate cancer cells produced morphological evidence of blastic reaction presented as an increased number of MG-63 osteoblasts and increase density of type I collagen around the sites of inoculation with PC-3 cells. The inoculated MCF-7 and ZR-75 cells decreased the density of type I collagen and number of osteoblasts and invaded the collagen gel around the sites of inoculation. The KLE endometrial cancer cells and cell-free media produced no reaction at the inoculation sites suggestive of cancer cell-specific interactions with osteoblasts in this system. The expression of uPA was remarkably higher at the inoculation sites of PC-3 cells as compared with those of the other cancer cells. Cathepsin D expression was higher at the sites of inoculation with KLE, MCF-7 and PC-3 cancer cells. MG-63 osteoblasts contained relatively low expression of uPA and cathepsin D. We conclude that this collagen gel system is a useful model for studying the morphological evidence of local invasion and osteoblasts reaction produced in response to local growth of metastatic cancer cell in vitro.