Biochemical events involved in both IgE-dependent and IgE-independent mediator release from basophils and mast cells were analyzed. The results revealed that bridging of IgE receptors activates a variety of membrane-associated enzymes, such as serine protease, phospholipase C, methyltransferases and adenylate cyclase, resulting in the stimulation of phosphatidylinositol (PI) turnover and a transient increase in both phospholipid methylation and intracellular cAMP. Mobilization of intracellular Ca2+ monitored by Quin-2 fluorescence is detected within 5 s after antigen challenge and appears to be the earliest intracellular change detectable after receptor bridging. Stimulation of PI turnover results in the generation of inositol 1,4,5-triphosphate (IP3) and of 1,2-diacylglycerol (DAG), which in turn activates protein kinase C. Evidence was obtained that the guanyl nucleotide (GTP)-binding protein Ni is not involved in the transduction of IgE-mediated triggering signals for mediator release. Although the sequence of enzyme activation following receptor bridging is not clear, the results suggest that the mobilization of intracellular Ca2+ is a crucial initial signal in the IgE-mediated activation of basophils and mast cells. In the mediator release induced by IgE-independent stimuli, enzymes involved in the mediator release are different from one stimulus to another. The results indicate the presence of multiple biochemical pathways for mediator release from basophils and mast cells.