Morphological changes of basophils stimulated by anti-IgE, anti-IgG and an specific antigen were observed in 62 asthmatic and 7 healthy subjects by direct counting. The percentage of anti-IgE induced reactive basophils determined by the direct counting method was similar to that by the smear method. Basophil reactivity to both anti-IgE and anti-IgG was low in the healthy subjects. Basophils from asthmatic subjects with a high serum IgE level reacted strongly to anti-IgE. The optimal concentration of house dust for observing reactive basophils was a 10(2)-fold dilution of the extract. The percentage of reactive basophils induced by house dust correlated with the skin end point, the RAST score and the bronchial challenge test in asthmatic subjects sensitive to the corresponding antigen. These results suggest that observation of morphological changes by the direct counting method is useful for in vitro detection of specific antigens.