The roles of Ca and G proteins in granule release from human eosinophils were examined using a patch-clamp technique in single cells. The morphological changes and the release of eosinophil peroxidase (EPO) from single cells were simultaneously observed. The intracellular application of Ca, and GTP-gamma-S induced fusion of EPO containing granules with the surface membrane which was associated with a marked increase in membrane capacitance. Ca alone caused a rapid granule release at an early stage of cell dialysis, but most granules still remained in a cluster. GTP-gamma-S alone caused a gradual degranulation. Northern blot analysis revealed the definite expression of smg p21 mRNA with no appreciable expression of smg p25A in human eosinophils.