Cancer is induced by a series of genetic alterations that lead to changes in the normal mechanisms controlling cell proliferation, differentiation, cell death, or genomic instability. The MCF-10F, a spontaneously immortalized human breast epithelial cell line, treated with benzo(a)pyrene and then transfected with the c-Ha-ras oncogene was used in these studies. The aim was to define the phenotypic alterations associated with the carcinogenic process. Carcinogen-treated and c-Ha-ras-transfected cells showed a progression of changes in the morphology as seen by transmission electron microscopy, anchorage-independent growth, invasiveness and capability of tumor formation in the SCID mice, as well as altered oncoprotein expression. Furthermore, these cells showed an increased expression of MDM2, Int-2 (FGF-3) and beta catenin in comparison to control MCF-10F as determined by fluorescence staining coupled with confocal microscopy. The MDM2, Int-2 (FGF-3) expressions were increased in cell lines transfected with the c-Ha-ras with or without carcinogen treatment as well as the tumor cell line derived from a tumor formed in the SCID mice in comparison to control cell line MCF-10F. However, beta catenin was only increased in the most aggressive tumorigenic cell lines in comparison with MCF-10F cell line and non-transfected ones. It can be concluded that malignant progression is a stepwise process and tumor growth occurs after a series of molecular events that parallel morphological changes indicative of cell transformation.