The permeability to horseradish peroxidase, (HRP, MW 40 kDa) increased approximately sixty-fold by decreasing the concentration of polymer (HEMA-MMA, hydroxyethyl methacrylate-methyl methacrylate) in triethylene glycol (TEG) from 10 or 11% to 9% (w/v), in microcapsules prepared by interfacial precipitation into phosphate buffered saline. Permeability was determined at the individual capsule level by enzyme-chromogenic substrate assay. With the lower polymer concentration, aggregates and the interconnecting structures formed large interstitial spaces and what appeared to be large pores on the external surface, Adding poly(vinyl pyrrolidone) (PVP) to the HEMA-MMA solution increased the capsule permeability, as much as 25-fold, apparently by increasing the external pore density (defined as the number of pores per unit surface area) without significant deviation in pore size and its distribution. Thus PVP is potentially useful for enhancing the permeability of these capsules which are used to immunoisolate cells used for transplantation. Measurements of molecular weight cut-off are needed before demonstrating that PVP/HEMA-MMA microcapsules are capable of immunoisolating the entrapped cells while maintaining high permeability to the materials essential to encapsulated cell survival and the bioactive cellular products such as insulin.