Stable and narrow distribution polyion complex micelles (PICMs) were prepared in an aqueous milieu through electrostatic interaction between a pair of oppositely charged block copolymers poly(N-vinylpyrrolidone)-block-poly(2-acrylamido-2-methyl-1-propanesulfo nic acid) (PVP-b-PAMPS) and monomethoxy poly(ethylene glycol)-block-poly(4-vinyl pyridine) (PEG-b-P4VP). The critical aggregate concentration (CAC), hydrodynamic size, and surface morphology of the prepared PICMs were characterized by fluorescence spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM), respectively. The resulting CAC and the average diameter of the PICMs were about 43 mg/L and 121 nm, indicating high structural stability of micelles and a size favorable for delivery of drug. In addition, the PICMs exhibited good biocompatibility using the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) assay with human embryonic kidney (HEK293) cells. All of these features are quite feasible for utilizing the PICMs as a novel intelligent drug delivery system. In order to assess its application in the biomedical area, the model drug folic acid (FA) was loaded into the micelles and the in vitro drug release behavior was investigated. We found that by manipulating the pH value and salt concentration of the release solution, it was possible to control the release rate of FA. (C) 2010 Elsevier Inc. All rights reserved.