A simplified method for measuring basophil histamine release in grass pollen hay fever patients has been developed. Leukocytes were challenged in vitro with extracts of Phleum pratense (timothy) and the release of histamine was determined indirectly as the residual histamine in the cell sediment. Several steps to purify histamine thus became superfluous and histamine was directly conjugated with o-phthaldialdehyde to form a fluorophore. The simplified method showed a basophil histamine content which was in accordance with results obtained by more specific methods. No difference in basophil histamine content was found between normal and allergic persons. For the histamine liberation assay blood could be adequately preserved for transport for 48 h at room temperature by adding cell culture medium. Basophil histamine release technique allows evaluation of cell sensitivity for determination of the degree of allergy as well as the level of blocking antibodies.