We examined histamine release and morphological changes in basophilic granulocytes from atopic subjects, in response to stimulation with antigen and Ca ionophore A23187. 1. Histamine release and a reduction in the rumber of basophils were more rapid and greater in extent at an early stage of antigen stimulation compared with Ca ionophore A23187 stimulation. 2. Morphological changes in basophils, represented by increased motility, in terms of an increased ratio of short to long axis diameter (L/S ratio), as well as the increased frequency of basophils with localized granules and those with pseudopods, were more often observed antigen stimulation than with Ca ionophore A23187 stimulation. In contrast, morphogical changes in which basophils appeared swollen, showing an increased mean cell diameter and an increased frequency of cells with 5 vacuoles or more were more predominant with Ca ionophore A23187 than with antigen stimulation. The results obtained here show that bridging of IgE receptors is essential to activate basophils and induce increased motility in these cells.