The immobilization of protein A and IgG by entrapment in polyacrylamide microbeads with a diameter of 1 μm was studied with the aim of optimizing the yield of immobilized protein. The microbeads were prepared by a modified emulsion polymerization technique. The small size of the beads resulted in an increase in immobilized protein available for interaction at the bead surface when compared with beads of larger diameter (100 μm). The increase was approximately proportional to the increase of the total bead surface. The increase in total bead surface was, however, accompanied by a slight decrease in the degree of immobilization. Optimal immobilization of protein A and IgG was obtained at monomer compositions of T-C 10-5 and 20-5, respectively (for definitions of T and C, see below). The fraction of immobilized protein available for interaction at the bead surface was almost constant over a wide range of protein concentrations tested. The yield of immobilized protein is generally rather low. The immobilization of IgG could be increased, by the presence of urea or by increasing the apparent molecular weight of IgG by addition of soluble protein A, thus forming soluble IgG protein A complexes. A large fraction of the non-immobilized protein (20–90%) could be recovered from the washings by affinity chromatography. The entrapment seemed to be more related to the molecular weight than to the hydrodynamic properties of the protein. The density of the microbeads was determined in Percoll and was found to be dependent on the total monomer concentration (T). At high concentrations of cross-linker (C) the apparent density was increased, owing to diffusion of the density medium into the large pores of the gel.