Polymorphonuclear leukocytes (PMN's) incubated three to eight minutes at 37°C in medium containing 1 × 10−6M of the ionophore antibiotic A23187 released their cytoplasmic granules into the extracellular medium. Transmission electron microscopy of treated cells showed microfilament bundles extending between adjacent granules within the cytoplasm and between granules and the plasma membrane. Tiny dense projections (beads) 8-12 nm in diameter were observed along segments of the cytoplasmic surface of the plasma membrane with a periodicity of 20–30 nm. These beads were observed on the plasma membrane only in the vicinity of intra- or extracytoplasmic granules. The structural relationships of the beads with the plasma membrane micro-filaments suggest they play a role in the process of ionophore-induced granule release from polymorphonuclear leukocytes.