THE role of microfilaments in secretion has been much debated1–4. If secretion involves a sliding filament mechanism similar to muscle contraction5, microfilaments must attach at least transiently to vesicle membranes. Indeed, there is evidence that contractile proteins are associated with secretory vesicles (chromaffin granules) of adrenal medulla6,7. Moreover, the outer (cytoplasmic) surface of secretory vesicles must provide ‘anchoring molecules’ for microfilaments. In skeletal muscle, the structural protein α-actinin located in the Z-line8 serves as such an anchoring molecule. An α-actinin-like protein was isolated from a variety of non-muscle cells9,10 and there is evidence that the same protein is attached to the inner (cytoplasmic) surface of the plasma membrane of non-muscle cells in association with microfilaments11–14. Using a monospecific antibody to porcine α-actinin (Fig. 1) we have demonstrated the presence of this protein in secretory vesicle membranes from chromaffin cells of the adrenal medulla and from platelets.