Members of the Fos family of AP-1 transcription factors (c-Fos, FosB, FosB2, Fra-1 and Fra-2) are able to form dimers with Jun proteins which bind to the regulatory sequences of target genes. As many proteases involved in tumor invasion are AP-1-regulated, we assumed that Fos family members might be important for invasion of mammary carcinomas. Therefore, we performed transient transfections with expression vectors for c-Fos, FosB, FosB2, Fra-1 and Fra-2, followed by matrigel invasion assays. Fra-1 transfection resulted in a 2-4-fold increase of invasive cells in both cell lines. In a less degree, the invasive potential of MDA-MB231 cells was stimulated by Fra-2, whereas MCF7 invasion was enhanced by c-Fos and FosB. By double-labelling immunocytochemistry, PAI-1 up-regulation was observed in cells transfected with c-Fos, Fra-1 and Fra-2 expression vectors, whereas MMP1 and MMP9 expression was not affected. Results of cotransfection with a MMP9 promoter construct and AP-1 expression vectors do not indicate a direct up-regulation of MMP9 expression by Fos proteins except a positive effect of c-Fos in MCF7 cells. In parallel, expression of Fos family members as determined by Western Blot analysis in 75 mammary carcinomas was correlated with MMP1, MMP9, PAI-1 and uPAR protein levels in the tumors. Interestingly, high FosB levels were significantly associated with MMP1 overexpression, whereas expression of c-Fos and phosphorylated Fra-1 correlated with MMP9 protein levels. Strong Fra-2 expression correlated with high levels of MMP9, PAI-1, the uPA/PAI-1 complex and early recurrence. These data indicate that Fos proteins, especially Fra-1, c-Fos and Fra-2, might be involved in invasion of breast cancer cells.