IgM-RF B cell precursors are abnormally overrepresented in "well differentiated" lymphoid monoclonal proliferations while data on less mature lymphoid malignancies are still awaited. This nevertheless suggests that RF activity plays a role in the transforming process perhaps by inducing constant stimulation of the precursor B cells. Despite the preferential use of similar VH and VL genes with little or no somatic hypermutations in both malignant B-cell CLL and nonmalignant mixed cryoglobulinemia, these proliferations do differ in CD5 membrane expression and in their clinical evolution. One possibility could be that CD5 glycoprotein is lost during maturation of the lymphocyte into a secreting cell as suggested by data on Waldenström's disease and the LES-CLL and by in vitro studies. Alternatively, CD5 expression could play an additional direct role in malignant transformation as suggested by recent data on the CD5 receptor ligand. Further data on the proliferating cells in both situations as well as on the genetic control of CD5 expression in B cells and its physiology should shed additional light on the mechanisms of B-cell malignancy.