The rodent S100-related calcium-binding protein, S100A4 induces metastasis in non-metastatic rat and mouse benign mammary cells and co-operates with benign-tumour-inducing changes in two transgenic mouse models, to yield metastatic mammary tumours. Co-transfection of the human gene for S100A4 with pSV2neo into the benign rat mammary cell line, Rama 37, yielded cells which expressed a low level of the endogenous S100A4 mRNA, and either high or undetectable levels of human S100A4 mRNA. The cells which expressed a high level of human S100A4 mRNA induced metastasis in the benign rat mammary cell line Rama 37 in an in vivo assay, whereas the cells which expressed an undetectable level of human S100A4 did not induce any detectable metastases. The primary tumours arising from the S100A4-expressing cells contained high levels of immunocytochemically-detected S100A4 and this high level of S100A4 and the metastatic potential were maintained when cells from a metastasis were re-injected into syngeneic rats. The results show that the human S100A4 possesses metastasis-inducing capabilities.