Two proteins with binding capacity highly specific for serotonin are present in the 100,000 g supernatant obtained from rat platelets: a glycoprotein and albumin. They were purified by means of (NH4)2SO4 fractionation, Sephadex sieve chromatography, and affinity column chromatography. The two proteins differed in most of their physical and chemical properties: (a) migration on 7.5% acrylamide gel of the complex (protein-Fe+2-[3H]serotonin); (b) molecular weight (sodium dodecyl sulfate gels); (c) carbohydrate reaction; (d) binding capacity and binding constants; (e) effect of reserpine; (f) heat stability; and (g) isoelectric point. However, they showed two similar properties: sensitivity to trypsin and dependence on Fe+2 for serotonin binding. The properties of both the glycoprotein and albumin differ considerably from those of serotonin binding protein of brain (SBP), which was not detected in platelets. Since brain serotonin binding protein is thought to be involved in storage of the amine, these results suggest that the storage form of serotonin in rat platelets is different from that in rat brain. These results also imply that the storage of serotonin in platelets may not serve as a model for the storage of serotonin in nerve terminals of the brain.