A model is presented for the potential distribution across a protein-coated microcapsule membrane. The adsorbed protein layer is assumed to be divided into three sub-layers. The outer sub-layer is rich in positive fixed charges and the medium sub-layer in negative fixed charges, while the inner sub-layer may be either charged or uncharged. Also, the microcapsule membrane is negatively charged. The ionic partition coefficients into the inner sub-layer are less than unity, while those into the other sub-layers are unity. This model explains well the accelerated degradation of poly(L-lactide) microcapsules with an adsorbed protein layer onto their surface (Makino et al., 1987 c).