The functionalization of bioactive principles is a relevant issue as it allows increasing their stability, to raise the hydrophilic properties of hydrophobic materials, to reduce the absorption from macrophages and the proteolytic degradation. For this reason, we have functionalized a model protein (Ribonuclease A) with polyethylene glycol (PEG5000). We have investigated the production of lipid/PEG particles incorporating this active principle by a supercritical fluid technology (particles from gas saturated solution, PGSS) to obtain solid micro- and nanoparticles. Runs were carried out to study the process conditions; thus, the effects of the operative variables, such as temperature, pressure and organic solvent, were optimized to obtain micrometric particles. The particles produced were characterized by static light scattering (SLS) to determine their mean and distribution size. The better operative conditions were employed to produce microparticles incorporating Ribonuclease A (RNAse) as both native and PEGylated form. The BCA test was applied to determine the maximum amount of protein incorporated in the particles and the in vitro release. The retained enzymatic activity of proteins after the PEGylation and after the micronization process was also determined by spectrophotometric evaluation.