Factors influencing the in vitro release of bovine serum albumin (BSA) from poly(butylcyanoacrylate) (PBCA) nanocapsules, such as the pH value, BSA loading, the polymeric nanocapsule walls and protein molecular weight, were investigated in detail. The BSA release rate was affected by the degradation rate of the polymeric wall and protein loading. For low molecular weight proteins, the initial burst release was faster than that of high molecular weigh proteins and got to equilibrium quickly. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis results showed that BSA encapsulated within PBCA nanocapsules did not suffer covalent aggregation or fragmentation during the initial days of in vitro incubation. For nanocapsules prepared by interfacial polymerization in waterinoil microemulsions, these findings were useful as a foundation for the development of nanocapsules with desired properties.