Spherical microporous reservoir-type microcapsules, fabricated using a W/O/W double emulsion technique with solvent evaporation and composed of 330 kD poly(beta-hydroxybutyrate-hydroxyvalerate (P(HB-HV)) (10.8% HV)/20% PCL II containing a range of bovine serum albumin (BSA) loadings, were incubated in Hank's buffer, pH 7.4, newborn calf serum (NCS), 1.5% pancreatin and synthetic gastric juice containing 10% pepsin over 30 days, and their percentage weight loss (PWL) and change in ultrastructural morphology monitored by gravimetry and stereoscan electron microscopy (SEM), respectively. The greatest percentage weight loss from microcapsules was observed after incubation in NCS and decreased in the order NCS > pancreatin > synthetic gastric juice > Hank's buffer. Only 5, 10 and 15% bovine serum albumin (BSA) loaded microcapsules incubated in Hank's buffer and synthetic gastric juice showed a significant increase in PWL with increasing percentage BSA loading. The overall sequence of changes in structural morphology due to biodegradation occurred at different rates in the different 'physiological' media. An initial increase in micropore diameter was followed by the coalescence of micropores to form macroporous pits (Hank's buffer). Further biodegradation in NCS, pancreatin and synthetic gastric juice was characterized by significant surface and bulk erosion. Only in pancreatin and NCS did biodegradation proceed to a loss of spherical shape and partial (pancreatin) and almost total (NCS) disruption of microcapsule structure after 30 days.