AbstractAnalysis of macromolecular fraction of platelet release products (PRPr) was undertaken after we observed that pretreatment of human neutrophils with macromolecules (>105daltons) involved in PRPr enhanced phagocytosis of IgG‐sensitized sheep red blood cells (IgG‐SRBC). Sephadex G‐200 gel filtration of PRPr revealed two macromolecular activators and two suppressors of neutrophilic phagocytosis of IgG‐SRBC: MAPP(s), macromolecular activator(s) of phagocytosis from platelets; MSPP(s) macromolecular suppressor(s) of phagocytosis from platelets. The estimated molecular weights of the respective MAPPs were 100 kdaltons to 140 kdaltons (kd) (s‐MAPP) and 200 kd to 290 kd (I‐MAPP) and those of the respective MSPPs were 60 kd to 100 kd (s‐MSPP) and 400 kd to 500 kd (I‐MSPP). It is suggested that an l‐MAPP molecule has at least an s‐MAPP molecule as a functional unit and that MAPPs and larger MSPP (I‐MSPP) might be glycoproteins, while smaller MSPP (s‐MSPP) seems to contain neither carbohydrates nor lipids. As far as the effects of serum and plasma on phagocytosis of IgG‐SRBC by neutrophils are concerned, the serum gave twice as much phagocytic activity as the plasma. Substantial MAPP‐like activities were observed In the serum and MSPP‐like activities were observed in both the serum and plasma.