Several costimulatory molecules play a key role in the differentiation of B lymphocytes and in T-B-cell interactions. In this study, we addressed the question of whether different receptors and counter-receptors may be expressed on malignant B lymphocytes from chronic B-cell malignancies. Using flow cytometry and reverse transcription PCR analyses, the expression of molecules belonging to the tumor necrosis factor receptor (TNFR) and tumor necrosis factor ligand (TNFL) families, as well as the expression of CD80 and CD86 molecules, was analyzed in normal B cells and in different chronic lymphoproliferative disorders of B-cell type, including B-cell chronic lymphocytic leukemia (CLL), mantle cell lymphoma, hairy cell leukemia (HCL), and HCL variant. Different patterns of expression of TNFR and TNFL superfamily molecules were demonstrated among B-cell malignancies. In particular, CD40 was commonly observed on all B cells (both tumor and normal), whereas its ligand (CD40L), which is usually undetectable on resting normal B lymphocytes, was expressed in CLL and HCL but not in other chronic lymphoproliferative disorders. CD27 was not shown in normal B cells, although it was present in all malignancies and with particularly high density in mantle cell lymphoma. CD70 was widely distributed on tumor B lymphocytes, but not on the CD5+ normal counterpart. CD30 was strongly expressed in HCL variant and weakly in B-cell CLL, whereas its ligand showed a wide pattern of expression, including all neoplastic and normal B cells. TNFR II (CD120b) and CD80 were distributed on neoplastic B cells from all groups, usually at an intermediate to high degree of intensity, whereas the CD86 molecule was present at lower intensity than CD80. Finally, reverse transcription PCR analysis confirmed the presence of CD40L, CD30, and CD30L mRNAs in those B cells expressing the corresponding membrane-bound proteins at low density. Our data indicate that TNFR and TNFL molecules are of use clinically both in differentiating B-cell malignancies from the normal counterpart (i.e., CD27, CD70, CD40L, CD30, and CD80) and in defining different chronic B-cell disorders (i.e., CD40L, CD27, and CD30). Interestingly, the observation that several receptors and their ligands (i.e., CD40/CD40L, CD30/CD30L, and CD27/CD70) can be expressed on the same cell suggests that these molecules play a role in initiating and maintaining the neoplastic process by mediating B-T and B-B interactions.