A new entity of polymer vesicle with a polyion complex (PIC) membrane, a PICsome, was prepared by simple mixing of a pair of oppositely charged block copolymers, composed of biocompatible PEG and poly(amino acid)s, in an aqueous medium. Flow particle image analysis revealed the formation of spherical particles with a size range up to 10 mum. Observation by dark-field and confocal laser scanning microscopes clearly confirmed that the PICsome has a hollow structure with an inner-water phase, in which FITC-dextran emitting green fluorescence was successfully encapsulated simply by the simultaneous mixing with the block copolymers. Confocal laser scanning microscopic observation and spectral analysis revealed the smooth penetration of a low molecular weight fluorescent dye (TRITC; MW = 443.5) emitting red fluorescence into the FITC-dextran encapsulated PICsome to give the PICsome image with a merged color of yellows, indicating the semipermeable nature of the PICsome membrane. The PICsomes showed appreciable physiological stability even in the presence of serum proteins, suggesting their feasibility in biomedical fields such as carriers of therapeutic compounds and compartments for diagnostic enzymes.