Structural changes in the extracellular matrix (ECM) are necessary for cell migration during tissue remodeling and tumor invasion. The matrix metalloproteinases (MMPs) and their inhibitors have been shown to be critical modulators of ECM composition and are, thus, crucial in neoplastic cell invasion and metastasis. Expression of MMP-2, -3, -9, -10, and -13 was investigated in human malignant melanomas (MMs) employing an indirect alkaline phosphatase conjugated immunocytochemical technique. Evaluation of the results was based on (a) the percent of neoplastically transformed cells/surrounding stroma that reacted positively and (b) a measure of staining intensity [graded from A (highest) to D]. The two forms of stromelysin, MMP-3 and -10, share 82% sequence homology, but exhibit differences in cellular synthesis and inducibility by cytokines and growth factors in vitro. Strong overall expression of MMP-3 and -10 was found in MMs, especially in the ECM adjacent to blood vessels. Positive immunoreactivity could be seen for these two MMPs in the ECM surrounding over 90% of the neoplastically transformed cells (++++), and the staining intensity was also the strongest possible (A,B). Focal (+), high intensity (A,B) staining could be detected for MMP-2, -9, and -13. Thus, it seems that the stromelysins are involved in the generalized growth and expansion of the neoplastic cell mass, while MMP-2, -9 and -13 are involved in the neoangiogenic and focal clonal selection and expansion phenomena associated with in situ tumor progression, invasion of the microvasculature, and metastasis.