The aim of the present study was to evaluate the in vivo effects of the RAR-alpha selective antagonist Ro 41-5253 on a xenograft animal model for breast cancer. Our observations indicate a lack of toxic side effects of the drug, even when used at high dosages. It is interesting to note that using Ro 41-5253 at dosages of 10, 30 and 100 mg/kg/die resulted in a slight, but significant inhibition of cell growth. The data obtained in this study represents the basis for a further evaluation of Ro 41-5253 anti-neoplastic activity on transgenic breast cancer animal models.