Subjects (72) were tested by skin-test (ST) and human basophil degranulation test (HBDT) with grass pollen extract (GPE). HBDT was done with a suspension enriched in basophils using a slide technique. Results were expressed by an index (DI) which is the percentage of basophils which apparently disappear (i.e., degranulated) in the presence of GPE. No less than 100 intact basophils (and often many more) were counted without GPE (control) for each subject, so that DI higher than 35% are significant (P < 0.001). DI were higher than 35% in 28 positive ST subjects for at less 1 of 3 dilutions of GPE (10-4, 10-6, 10-8). In the 5 subjects with false-negative results, 3 had a weak ST response. In 39 subjects with negative ST, DI were higher than 35% in 2 subjects and only for 1 dilution (10-4). Concordance of results of HBDT and obviously positive ST reached 94%. In 33 positive ST subjects, a significant correlation became evident between DI (at 10-4 and 10-6) and flare diameter and between DI (at 10-4) and wheal diameter. HBDT appeared to be a suitable and simple technique for routine study of reaginic allergies.