To clarify the mechanism of human basophil activation via IgG receptors, calcium mobiliza-tion in response to IgG antibodies was analyzed in the human basophilic leukemia cell line KU812-F, using flow cytometry. KU812-F cells as well human basophils selectively expres-sed the FcγRⅡ subtype of IgG receptor. After stimulation with aggregated IgG, an obvious increase in ［Ca(2+)］i was observed, but the increase was completely inhibited by Ⅳ. 3 (antiFcγRⅡmAb). Moreover, Ⅳ. 3 elicited a［Ca(2+)］i rise only when cross-linked on the cell surface with anti-mouse IgG. No significant histamine release was observed after any IgG stimulation and the biologic function of the FcγRⅡ-induced ［Ca(2+)］i rise remains unclar. These findings suggest that the cross-linking of FcγRⅡ in KU812-F cells induces signal transduction events and initiates cell activation with the exception of histamine release, and human basophils also may be activated in vivo by IgG antibodies.