In the majority o f chronic lymphocytic leukemias (CLL), proliferative B lymphocytes are thought to be frozen in vivo at a given maturat ion stage (1). Recently, several studies have emphasized that CLL cells could be driven to different iate into plasma cells in vitro (with or without proliferation) upon t r iggering by mitogens (2, 3, 4) or T cel l -der ived factors (5, 6). Th u s far, howeve...