The specific objective of this study was to determine if Fgf-3 gene expression is mediated by dietary fatty acids and to confirm mouse mammary tumor virus infection. It is well known that dietary linoleic acid enhances growth and dietary stearic acid inhibits growth of mammary tumors. Tumor RNA was extracted from female strain A/St mice fed one of four diets. A radioactively labeled anti-sense RNA probe was generated, invitro, from isolated and purified pFgf-3c (int-2c clone contained in the vector pSP65). The Fgf-3c probe was hybridized to extracted tumor RNA using the ribonuclease protection assay.Electron microscopy confirmed MMTV infection by visualization of type A and B particles in tumor tissue. Expression of Fgf-3c, qualified by RNase protection assay, ranged from 0.02 to 5.89 (relative band density) in all of the diet groups. A positive association between Fgf-3c expression and weight was observed among the tumors of the SA-1 diet (R = 0.947). The SF, SF-1, and PA experimental diets, individually, did not appear to show strong correlation with respect to tumor size. Fgf-3 expression was less in small tumors (<275 mg) and enhanced in large tumors (>275 mg) (p<0.05). / Department of Biology