We have used reconstituted basement membrane molecules which have formed into barriers in order to investigate the invasive potential of malignant bone and soft tissue tumour cells in vitro. A number of cell lines established from human malignant tumours demonstrated a high degree of invasiveness, although fibroblasts showed no ability to penetrate the basement membrane barrier. H-ras oncogene transfected cells into the fibroblasts were much more invasive than the parent lines. Primary cultures of malignant tumour cells demonstrated invasiveness, while those of nonmetastatic cells and fibroblasts did not. The binding of tumour cells to laminin in the basement membranes was found to induce secretion of collagenase and motility which are crucial factors for invasion. A synthetic peptide, Tyr-Ile-Gly-Ser-Arg, was able to suppress the invasiveness of HT1080 human fibrosarcoma cells, and also reduced lung colonisation in vitro. The results suggest that the in vitro assay was useful, firstly to determine the invasive potential, secondly to investigate the mechanism of invasion, and finally to development treatment against invasion and metastases.