In this chapter, the authors review their studies characterizing the nature of the some types of helper activities and the roles they may play in B cell activation. The focus of their approach in these studies has been the development of an extensive panel of T cell hybridomas. These hybridomas offer a number of advantages for such studies, including rapid growth in the absence of filler cells and exogenous factors. As part of their effort to characterize the nature of the nonspecific factors present in mitogen Concanavalin A, the authors began the production of a panel of T cell hybridomas. Many studies have investigated the role of interleukins in B cell activation. In virtually every system studied to date, nonspecific factors have been without apparent effects in the absence of additional activating signals provided by mitogens, antiimmunoglobulin antibodies, or antigen plus la restricted helper T cell signals.