Abstract
The preparation of a fraction containing highly purified chromaffin granules from the bovine adrenal medulla is described. The fraction contains an adenosine-triphosphatase activity that is stimulated by Mg2+ and that cannot be explained by contamination with mitochondria or microsomes. It is suggested that the adenosine-triphophatase activity is related to the uptake of catecholamines by the chromaffin granules.