Morphological changes of basophil granulocytes from 5 atopic asthmatics after addition of specific antigen (house dust) were observed with a phase-contrast microscope. The motility of basophils from atopic asthmatics was less active than that of basophils from healthy subjects. The motility of basophils from healthy subjects did not change after the addition of antigen. On the other hand, the motility of basophils from atopic asthmatics significantly increased after addition of antigen, and some basophils showed degranulation. Basophils with oriented movement resembling that of neutrophils (A(2) type) characteristically appeared in atopic asthmatics after addition of antigen. No degranulation was observed in basophils from healthy subjects. Basophils showed a decrease in motility before becoming swollen, at which time the incidence of A(3) type basophils, with movement resembling that of monocytes, characteristically increased. Swollen basophils (B from) were stained variously from poor to well by toluidine blue. The mean diameter of basophils was larger among the poorly stained cells than among the well stained cells.