This short review of our own work presents two aspects of the studies on multiple myeloma (MM) in an animal model--the aging C57BL/KaLwRij mouse: 1. the immunological/biological aspect of the development of monoclonal B-cell proliferative disorders, the so-called monoclonal gammopathies (MG), and 2. the use of the mouse myeloma of the 5TMM lines for studies on the etiology/pathogenesis of MM and for developing new ways of treatment of this disease. Our research revealed that there are at least four major mechanisms in the development of MG. Many of the results were confirmed in clinical studies and lead to a new classification of MG according to their biology and possible pathogenesis. Most of MG can be classified into one of the following categories: 1. B-cell malignancies, 2. B-cell benign neoplasia, 3. MG due to an immunodeficiency with T/B cell imbalance, and 4. Antigen driven MG.