Novel, injectable, and sprayable nanometer size hydrated silica particles encapsulating high molecular weight compounds such as [I-125]tyraminylinulin (mol wt 5 kD), FITC-dextran (mol wt 19.6 kD), and horse radish peroxidase (mol wt 40 kD) have been prepared and characterized. The size of these particles is always below 100 nm in diameter and the entrapment efficiency was found to be as high as 80%. The entrapped compounds show practically zero leachability for more than 45 days. Enzymes entrapped in these particles show Michaelis-Menten kinetics and the catalytic reaction takes place only after the diffusion of substrate molecules into the particles through the pores of the silica matrix. Peroxidase entrapped into silica nanoparticles shows higher stability toward temperature and pH changes compared to free enzyme molecules.