Effect of dibutyryl cyclic adenosine monophosphate (dbt cAMP) on alkaline phosphatase (APase) of mitogen stimulated murine B lymphocytes was studied. Addition of dbtcAMP to lipopolysaccharide (LPS) stimulated B cells enhanced APase activity in a dose dependent and synergistic manner. dbtcAMP also stimulated the proliferative response of LPS treated B lymphocytes. On the other hand, when B lymphocytes stimulated with anti-immunoglobulin (anti-Ig) were treated with dbtcAMP neither DNA synthesis nor APase activity was enhanced. These results suggest that cAMP is a potent synergistic activator of APase in B lymphocytes committed to proliferation.