CD81 (TAPA‐1) is a member of the transmembrane 4 superfamily (TM4SF) which is expressed on the cell surface of most cells of the body throughout their cellular differentiation. It has been recognized in several cell surface complexes of lymphocytes, suggesting that it may have diverse roles in lymphocyte development and activation regulation. Mice with a CD81 null mutation revealed normal T‐and conventional B‐cell development, although CD19 expression on B cells was dull and B‐1 cells were reduced in number. However, both T and B cells of mutant mice exhibited strikingly enhanced proliferation in response to various types of stimuli. Interestingly, while proliferative responses of T cells following T‐cell antigen receptor (TCR) engagement was enhanced in the absence of CD81, B‐cell proliferation in response to B‐cell antigen‐receptor (BCR) cross‐linking was severely impaired. Despite these altered proliferative responses, both tyrosine phosphorylation and intracellular calcium flux in response to cross‐linking of cell surface antigen receptors were normal in mutant mice, reflecting apparently normal initial signaling of antigen receptors. In conclusion, though CD81 is not essential for normal T‐and conventional B‐cell development, it plays key roles in controlling lymphocyte homeostasis by regulating lymphocyte proliferation in distinct manners, dependent on the context of stimulation.