The in vitro carcinogenesis model derived from organ culture of the whole mammary glands of BALB/c female mice will be used to study the transforming action of the environmental carcinogens, benzo[a]pyrene (B[a]P) and N-diethylnitrosamine (DENA). Appearance of the potentially neoplastic nodule-like alveolar lesions (NLAL) in the mammary glands in vitro after exposure to B[a]P or DENA will be used as a morphological marker for preneoplastic transformation. Carcinogenecity of the mammary epithelial cells transformed in vitro will be confirmed by the ability of cells to produce mammary carcinomas after transplantation into gland-free mammary fat pads of syngeneic virgin mice. The unique organ culture model of mammary cell transformation also will be used to study the mechanics of chemopreventive action of the dietary compounds, a new retinoid, Beta-carotene and selenium, during the neoplastic process induced by DMBA, B[a]P or DENA. Influence of the chemopreventive agents on the molecular events such as cellular accumulation of the electrophilic reactants, DNA alkylation and DNA repair activity initiated by the carcinogenic chemicals will be examined. Attempts will be made to ascertain whether influence of the chemopreventive agents on the molecular events of "initiation" may alter morphological expression of the transformed cells as NLAL in the glands in vitro and/or in the mammary tumors in the mammary fat pads in vivo. Studies also will include determination of the influence of the chemopreventive agents during hormone mediated "promotion" of the initiated (transformed) cells, both in vitro and in vivo. These studies will examine influence of chemopreventive agents at the specific stages of transformation between normal -- preneoplasia and preneoplasia -- neoplasia. The results should provide elucidation o the mechanics of chemopreventive action of these dietary compounds.