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SUMMARY1. The distribution of the heparin-neutralizing factor (platelet factor 4, PF4) in subcellular organelles of blood platelets of rabbits and man was investigated.2. In both species the organelles storing 5-hydroxytryptamine (5-HT storage organelles) contained only trivial amounts of PF4.3. In contrast, the content of PF4 was highest in the subcellular fractions rich in a-granules.4. In conclusion, PF4 is probably localized in the a-granules and there.-fore the platelets contain at least two types of organelles (5-HT organelles and a-granules) capable of releasing their contents in response to the same stimuli, such as exposure to collagen, thrombin, etc.