The Nurses Health Study indicated that consumption of common bean reduced risk for breast cancer. This observation was replicated using a well characterized rodent model for mammary carcinogenesis and the effect of common bean was shown to be dose dependent. However, when low molecular weight fractions of bean were evaluated for growth inhibitory activity against established breast cancer cell lines, no inhibition was observed. Given that in vitro screens for anticancer activity are designed to detect antiproliferative and/or proapoptotic effects that decrease cell number, we reasoned that it might be possible to create a cell number accumulation assay for screening foods such as common bean in vivo by using an oncogene driven model for breast cancer. The advantage of this approach is that the system detects effects in the host that may be mediated by the gut microbiome. Here we report the characterization of an in vivo mammary cell accumulation assay driven by the PYMT oncogene and show that bean feeding reduced the accumulation of cells in developing mammary pathologies by 40% (p<0.01). As a potential mechanism, we also report the impact of bean consumption on the gut associated microbiome as a potential mediator of cancer preventive activity. Based on 16s rRNA sequencing of genomic DNA extracted from the cecum, significant differences were observed between common bean and the control diet in microbial phylogenetic diversity (4.25 vs 4.98, respectively, p=0.009), beta diversity (Bray-Curtis Permanova, p=0.008), and abundance for various detected taxa (84 differed significantly at FDR <0.05). Predicted functional activity of the metagenome (evaluated by PICRUSt/STAMP) will be reported.