Members of the Tumour Necrosis Factor-Receptor (TNFR) family play an essential role in the control of lymphoid cell growth and differentiation. The ligand of one of its lymphoid-specific members, CD27, was recently characterized as CD70, a type II transmembrane molecule with homology to TNF that is expressed on activated T and B cells. Ligation of CD27 by its natural ligand generates a potent costimulatory signal for cytokine production and proliferation of activated T cells. In contrast to normal B cells, where CD27 expression is confined to germinal centre cells and to a small subset of circulating B lymphocytes, CD27 expression is found on a large array of distinct B-cell neoplasia. Here, we review recent data on the expression and function of TNFR family members on normal and malignant lymphocytes and propose a role for CD27-CD70 interaction in B-cell development.