Major components of the secretory organelle of bovine adrenal medullary cells, the chromaffin vesicles, are the acidic protein chromogranin A, catecholamines and Ca²⁺. The binding of Ca²⁺ to chromogranin A has been established. To study the interaction between chromogranin A and catecholamines and its dependence on Ca²⁺ we immobilized chromogranin A to a newly raised monoclonal antibody. It is shown that chromogranin A can bind (i) about 0.5 mol catecholamines per mol in a non‐calcium‐dependent manner and (ii) about 5 mol per mol in the presence of calcium. These results further support the notion that chromogranin A may act as a secretory granule‐condensing protein.