When isolated from lysates of the RPMI 8866 B lymphoblastoid cell line, the CD23 antigen was found to be present be in two forms corresponding to the intact 45-kDa membrane species and to a 25-kDa product that is more usually found as a released fragment in the extracellular medium. By contrast with preparations of extracellular species of CD23 which were seen to be variable in their biological activity, cell-associated CD23 was consistently mitogenic both for autogenous transformed B lymphoblasts and for pre-activated normal B cells. Addition to the normal cocktail of protease inhibitors of N alpha-tosyl-L-lysine chloromethylketone (TLCK), which has selectivity for trypsin-like serine proteases, resulted in preparations of CD23 from RPMI 8866 cell lysates that were exclusively in the 45-kDa intact form; such material retained full and reliable activity in the biological assays. The implications of these observations for the autocrine control of B lymphocyte growth are discussed.