529 Background: We previously reported the association between a five monoclonal antibody test staining p53, NDRG1, SLC7A5, CEACAM5 and HTF9C and recurrence-free interval (RFI) in 711 ER+ N- breast cancer patients from Tamoxifen (Tam) arm of the NSABP trials B14 and B20 (SABCS 2006). In this study, we examined interaction between the test and chemotherapy in the B20 trial. Subjects and Methods: Tissue array sections from B20 paraffin blocks were stained using standard IHC protocols (N=457). Pre-defined scoring rules and cut- points were applied. RFI was defined as time from entry to any local, regional or distant recurrence. Log-rank test was applied to assess the effect of chemotherapy for each risk stratum pre-defined by this IHC test. Interaction between risk strata and treatment was assessed by the likelihood ratio test in a Cox model with age and clinical tumor size adjusted. Results: The IHC test identified high and low risk groups that both showed significant improvement upon treatment with cytotox...