Understanding the regulation of antibody synthesis is central to any effort to control the disordered immune responses which underlie many autoimmune disorders such as myasthenia gravis. I will discuss here the mechanisms involved in the stimulation of B lymphocytes, the cells which are the precursors of antibody-secreting cells. Emphasis will be placed on the activation of B cells from their resting state, the regulation of their proliferation, and some features of their differentiation into antibodysecreting cells, including a brief consideration of the problem of class switching. B lymphocytes are derived from precursors in hematopoietic tissue. Much has recently been learned about the biochemical and genetic events that control the assemblage of immunoglobulin (Ig) genes in B-cell precursors.'From this information, a picture is emerging of the mechanisms through which antibody diversity is achieved. Much less is known about the regulation of pre-B-cell growth and development. An important recent contribution to this field has been the introduction of a system through which pre-B cells may be cultured on monolayers of bone marrow stromal cells. In such cultures, pre-B cells proliferate and undergo some differentiation into mature B cells. The stromal cell monolayer appears to be critical to these processes, presumably reflecting the production of growth-regulatory substances by stromal cells. Efforts to characterize the stromal cells and to identify and purify the growth factors are now in progress. Work of this type promises to yield information of major significance to our ability to regulate the overall level of B-cell production.