A case with features of both multiple myeloma and non-Hodgkin's lymphoma at the moment of diagnosis is presented. The patient had lytic bone lesions and biclonal gammopathy, IgM kappa and IgA kappa. In the bone marrow biopsy, there was a diffuse infiltration by atypical plasma cells coexisting with an interstitial and nodular infiltration by poorly differentiated lymphoid cells. Immunofluorescence studies showed positive staining with alpha and kappa antisera in the cytoplasm of plasma cells and with mu and kappa antisera on the surface of lymphoid cells. After the beginning of chemotherapy, the IgA kappa monoclonal protein disappeared and the IgM kappa monoclonal protein remained constant.