Flow cytometry is a highly valuable method to monitor minimal residual disease (MRD) and evaluate the depth of complete response in bone marrow of multiple myeloma after therapy. An optimized antibodies panel validated by Euroflow detect minimal numbers of plasmatic cells in multiple myeloma (10-2 – 10-4). For the measurement of these specific population of cells with the presence and absence of a drug and in different concentration we can valuate drug efficacy in these very resistant subset of cells.