The monomer N-vinyl alpha -phenylalanine was prepared by interacting acryloyl chloride with the methyl ester protected alpha -phenylalanine. This monomer was used to form noncrosslinked and crosslinked poly(N-vinyl alpha -phenylalanine) microspheres. The noncrosslinked particles were formed by dispersion homopolymerization of this vinyl monomer in different mixtures of water/2-propanol. Under these polymerization conditions, an equilibrium between insoluble polymer composed of poly(N-vinyl alpha -phenylalanine) microspheres and a soluble polymer always existed. The effect of the ratio, by volume, of water/2-propanol on the insoluble (microspheres) and soluble polymer-e.g., molecular weight, molecular weight distribution, yield, and microsphere diameter-was elucidated. Poly(N-vinyl alpha -phenylalanine) microspheres were also formed by decreasing the solubility of the soluble polymer, via addition of the soluble polymer solution into pure water. This process has also been used for microencapsulation purposes. As a model, microencapsulation of fluorescein isothiocyanate within these microspheres has been demonstrated. To avoid the formation of the soluble polymer, and thereby to form insoluble polymer only, crosslinked microspheres were produced by dispersion copolymeriztion of N-vinyl alpha -phenylalanine with the crosslinker monomer: divinyl benzene or ethylene glycol dimethacrylate. The effect of the molar ratio [monomer]/[crosslinker] on the size of the microspheres was elucidated. Carboxylate functional microspheres were formed by basic hydrolysis of the ester protected crosslinked microspheres. The carboxylate functional groups of these microspheres were then used for covalent binding of proteins, e.g., trypsin, to the particles. The stabilization effect of the immunobilized trypsin against trypsin inhibitor has been clearly demonstrated.