The ability of the B cell to distinguish among regulatory species is probably determined at multiple levels. These include differential receptor expression, differential receptor coupling to specific second-messenger generating systems, and differential cellular programming for specific responses to specific second messengers. This chapter reviews the transduction mechanisms which may be operative during B cell activation. It discusses mechanisms which are potentially operative and presents findings regarding signal transduction via membrane immunoglobulin and receptors for B cell growth factor 1 (BCGF1). The chapter focuses on the role of protein phosphorylation in transmembrane signaling. A number of studies have been directed toward the role of calcium, cyclic AMP (cAMP), and cGMP, presumably acting via protein kinases in lymphocyte activation. The chapter examines the effect of semipurified BCGF1 on phosphatidylinositol metabolism, intracellular free calcium levels, and membrane potential. It provides a series of preliminary studies in an effort to define the biochemical basis of signal transduction by BCGF1 receptors.