A suitable model analyzing the behavior of well-differentiated squamous cell carcinoma has not yet been established. We tried to establish such a system using a reconstructed oral mucosa, in which T3M-1 squamous cell carcinoma cells were cultured on 3T3 fibroblast-containing collagen gel. Fibroblasts promoted the stratification and keratinization of T3M-1 cells. During growth, the Ki-67 index of T3M-1 cells with fibroblasts was higher than that of T3M-1 cells alone. Fibroblasts increased the expression of involucrin, a differentiating marker of keratinocytes, in T3M-1 cells. They also promoted the invasion of T3M-1 cells into the gel. When T3M-1 cells alone were cultured in a fibroblast-conditioned (FC) medium, the fibroblast-induced phenomena mentioned above were almost replicated. In addition, epidermal growth factqr (EGF) promoted T3M-1 cells growth, but not the invasion. cDNA microarray analysis showed that FC medium increased the expression of EGF receptor and several other mRNAs of T3M-1 cells. The data suggest that T3M-1 cells, under cancer-stromal fibroblast interaction, undergo invasive growth with their well-differentiated squamous phenotype, and that this interaction may be mediated partly by soluble molecules (e.g., EGF) in an autocrine or paracrine pathway. Our system will probably provide a useful model for analyzing the biological behavior of well-differentiated squamous cell carcinoma.