We have developed a microtiter assay for evaluating basophil spontaneous adhesion to extracellular matrix (ECM) proteins exemplified by fibronectin and cytokine induced basophil adhesion to bovine serum albumin (BSA). The percentage of basophils adhering to either ECM or BSA was quantified by the histamine content of the adhering basophils. The spontaneous adhesion to fibronectin was higher than to laminin and collagen type I. Both spontaneous adhesion to fibronectin and interleukin-3 (IL-3), interleukin-5 (IL-5), granulocyte/macrophage colony stimulating factor (GM-CSF) induced adhesion to BSA increased with time between 5 and 45 min. The histamine release in both spontaneous and induced basophil adhesion was lower than 3.1%. This microtiter assay is simple and reproducible and can be applied for basic and clinical studies using a limited number of partially purified basophils.