We investigated three clonally related human keratinocyte cell lines of different biological behaviour, HaCaT (non-tumorigenic), A5 (benign, tumorigenic) and II-4RT (malignant, tumorigenic), with regard to the cell-associated localization of the major MMPs -1, -2, -3, -9, -10 and -11. These observations were correlated with the pattern of cytokeratins (CK) 10, 13 and 14 which served as markers for cellular. In all three cell lines, we detected immunohistochemically various MMPs within the cytoplasm of the tumor cells, however, with differences between the MMPs and the various cell lines. MMP-1, -10 and -11 were strongly and equally present in all cell lines tested. MMP-2 was seen only faintly in all cells. In contrast, MMP-3 was only faintly seen in the cytoplasm of the non-tumorigenic, more extensive in the benign A5 cells, but strongly and extensive in the malignant II-4RT cells. MMP-9 was also seen in increasing intensity corresponding to the tumorigenicity of the three cells lines, however, with less intense staining than MMP-3. A semiquantitative immunoreactive score confirmed these observations. In parallel, the CK pattern indicated advanced cellular maturation of HaCaT (as seen by expression of basal CK 14 and suprabasal CKs 10 and 13). In the A5 cells a reduced expression of suprabasal CKs-10 and -13 indicated lesser maturation. The malignant II-4RT cells revealed even fewer cells with suprabasal keratinocyte differentiation. Our study clearly confirms differences in the concentration of cell-associated MMPs. These analyses parallel and supplement our previous biochemical studies. In addition, we provide circumstantial evidence that the expression of several major MMPs is associated with changes in the maturation pattern as evidenced by expression of CKs.