To identify genes whose expression is down modulated in the process of metastasis, gene expression was analyzed in cell lines derived from Dunning R-3327 rat prostatic tumor sublines. A complementary DNA (cDNA) library from the anaplastic nonmetastasizing subline AT-1 was used for a differential hybridization analysis, using probes derived from mRNAs of the AT-1 and the metastasizing MAT-LyLu subline. In this way 14 cDNA clones were isolated representing 6 differentially expressed genes. The expression levels in a panel of tumor sublines measured with these cDNA clones were tested for correlation with the anaplastic non-metastasizing phenotype. One cDNA clone, designated pSE-1, whose expression was high in all tested sublines with that phenotype, appeared to represent the gene for fibronectin. To further investigate the down modulation of this gene, we studied its expression in AT-2 (anaplastic, nonmetastasizing tumor) and lines derived therefrom that exhibited a high metastatic potential after transfection with the v-Ha-ras oncogene. In the genetically manipulated metastasizing tumor sublines, fibronectin mRNA levels were approximately 4- to 8-fold lowered compared to the nonmetastasizing parental AT-2 line.