AbstractStimulation of the phagocytic activity of leukocytes by two glycoproteins, the macromolecular activators of phagocytosis 1‐MAPP and s‐MAPP, involved in platelet release products (PRPr), was not observed when platelet concentrate (PC), stored longer than 48 h, was used as a platelet source. The supernatant of the PC (Sup) acquired this activity during storage. Salting out with ammonium sulfate (AS) revealed that the phagocytosis‐enhancing activity was recovered in the supernatant of Sup from PC with 60% saturated AS, and the reverse activity, phagocytosis suppression, was observed in the precipitate with 30% AS. Suppressing activity was observed in PRPr prepared from both fresh and 48‐h‐old PC. The phagocytosis‐enhancing activity observed in the Sup from the 48‐hour PC was characterized by Sephadex G200 gel filtration and consisted of two peaks (300 and 150 kD), matching these of 1‐MAPP and s‐MAPP in the PRPr prepared from fresh platelets. These results indicate that 1‐MAPP and s‐MAPP escape from platelets into the plasma during storage, whereas the suppressing activity is retained in platel