Human basophils were developed in suspension culture of mononuclear cells of cord blood in the presence of conditioned medium of phytohemagglutinin-stimulated human T cells, from which interleukin 2 had been eliminated. Approximately 50-90% of cells recovered after 2-4 wk of culture were basophil granulocytes, which bear receptors with high affinity for human IgE. Sensitization of the cells with human IgE followed by challenge with anti-IgE resulted in the release of histamine and arachidonic acid (AA). In both the cultured basophils and human lung mast cells, bridging of cell-bound IgE with anti-IgE induced a transient increase in phospholipid methylation and intracellular cyclic AMP (cAMP), and these processes were followed by Ca2+ uptake and release of both histamine and AA. As demonstrated in rat mast cells, evidence was obtained that the activation of a proteolytic enzyme and phospholipid methylation induced by the bridging of IgE receptors are involved in the subsequent increase in AMP, and are an essential step for transduction of triggering signals for mediator release.