Morphological changes in human basophils from 11 atopic asthmatics were observed in whole blood smears with respect to histamine release after the cells were challenged by Ca. ionophore A23187. A decreased number of basophils and granules in the cells as well as increase in cell diameters were observed upon histamine release after stimulation with Ca. ionophore A23187. Compared to cell reactions to antigen or anti-IgE, Ca. ionophore A23187 produced both more gradual morphological changes and histamine release and induced greater increases in the diameter of the basophils. No change in the ratio of the short to long axis diameter was observed after stimulation with Ca. ionophore A23187. The results suggest that basophils do not show any increased mobility upon stimulaion with Ca. ionophore A23187, though antigen and anti-IgE stimulation induces increased motility in these cells.