Summary Platelet factor 4 was present in normal amounts in patients whose platelets are deficient in the storage pool of adenine nucleotides and in the electron–dense granules in which these substances are stored. These findings suggest that PF–4 may be stored in other granules. The release of PF–4 by collagen and epinephrine, however, was decreased in these patients and these findings suggest that the storage pool of ADP may, in addition to being released, play a role in the release mechanism. The findings in patients with thrombasthenia indicate that this is not related to the ability of storage pool ADP to aggregate platelets.