For a better insight into the mechanisms determining the recruitment of human basophilic granulocytes from the circulation to sites of allergic reactions, we studied the homotypic aggregation of these cells. The aggregation was studied with > 95% pure basophil suspensions obtained from peripheral blood in a double-color flow cytometric analysis. Homotypic aggregation was induced by treatment of the basophils with anti-IgE, house dust mite allergen, the chemoattractant FMLP, PMA, or IL-3. The aggregation by anti-IgE was, in part, mediated by interactions with Fc gamma R-II as indicated by 43 +/- 15% inhibition after pretreatment with CD32 antibodies. The aggregation was mediated by beta 2-integrins, as was shown by inhibition of the response by CD18 antibodies. The aggregation induced by anti-IgE, allergen, and PMA displayed comparable kinetics (t1/2 max, 3 to 4 min), in contrast to the degranulation of basophils. FMLP induced the most rapid response (t1/2max, 1.6 min). Inhibition of protein kinase C by staurosporine resulted in a strong (> 90%) inhibition of the PMA-induced aggregation, whereas the FMLP-induced aggregation was more than doubled (from 11.7 +/- 1.9 to 24.4 +/- 1.9%). Staurosporine did not affect the extent of the anti-IgE-induced aggregation, but it induced a retardation of congruent to 10 min. In most experiments, no clear correlation was found between degranulation and aggregation of human basophils. Most strikingly, IL-3 did not induce degranulation but did induce aggregation. Thus, the homotypic aggregation response of human basophils is induced by intracellular signals not necessarily leading to degranulation. This might be important in the physiologic appearance of basophils at sites of allergic late-phase responses or inflammation.