Eristostatin, a low molecular weight polypeptide (MW 5725), was encapsulated within two biodegradable poly(lactic acid-glycolic acid) microspheres. In vitro release profiles from the microspheres exhibited fast initial release for up to several days, followed by very slow or no release. The later stage of the slow release was due to protein aggregation within the microspheres. A simple, noninvasive method to detect the protein aggregation within the microspheres has been developed : extraction of radiolabeled protein from the microspheres by a DC electric field into a polyacrylamide gel and subsequent exposure to a gamma-ray sensitive film. Direct application of the degrading microspheres onto the sample loading zone of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) could successfully extract the monomeric and oligomeric proteins into the autoradiogram of a gel slab, while unextractable protein aggregates within the microspheres could be directly visualized in their loading position.