The intracellular ratio of adenosine 3',5'-cyclic monophosphate (cyclic AMP) to guanosine 3',5'-cyclic monophosphate (cyclic GMP) may control the developmental pathway followed by antibody-forming cell (AFC) precursors. The evidence for this is derived from several different types of experiments. First lipopolysaccharide (LPS) which is mitogenic for B lymphocytes, stimulates rapid, transient changes in intracellular levels of cyclic GMP but not cyclic AMP when added to mouse spleen cultures. Cyclic GMP itself stimulates DNA synthesis in these cultures, suggesting that the intracellular changes in cyclic GMP levels are involved in the mitogenic signal delivered by LPS to cells. The absolute amounts of cyclic nucleotides may vary widely in different cells under various conditions, however, the intracellular ratio of cyclic AMP to cyclic GMP is always high in nondividing cells and low in dividing cells. AFC precursors appear to respond to antigen in the absence of T-cell activity by inactivation (1-7). In the response to antigen in the presence of specific T cells, precursor cells proliferate and mature to AFC. Raising intracellular levels of cyclic AMP inhibits cell proliferation and leads to precursor cell inactivation (14, 15). It is suggested that the interaction of antigen with immunoglobulin receptors on the surface of precursors cells leads to the stimulation of adenylate cyclase activity and initiates the inactivation pathway. Since cyclic GMP stimulates immune responses in T-cell-depleted cultures (14, 15) and increasing cyclic GMP levels appear to be involved in the delivery of a mitogenic signal to cells, it is suggested that T-helper cells deliver a signal to precursor cells via the stimulation of guanylate cyclase to initiate the inductive pathway. It is suggested that it is the intracellular ratio of cyclic AMP to cyclic GMP that regulates the fate of precursor cells, not the absolute level of one cyclic nucleotide.