Regulation of human B cell responses is a complex process involving the activities of a variety of cells and cytokines. There are important differences between the regulation of human and murine B lymphocytes, especially with regard to the action of IL2 and IL4. In man, IL2 appears to play a central role in regulating B cell activation, proliferation and differentiation, whereas IL4 plays a regulatory role by inhibiting initial B cell activation. Above and beyond providing regulatory cytokines, activated human T cells can also directly stimulate B cell differentiation by a process that requires cellular contact. A better understanding of the nature of signal transduction during interactions between B cells and activated T cells and provided by the various cytokines should provide additional insight into the regulation of B cell function in man.