Poly(ethylene oxide)-b-poly(epsilon-caprolactone) (PEO-b-PCL) block copolymers self-assemble into micelles in aqueous solution. We have examined whether these micelles can internalize into P19 cells in vitro. Fluorescently labeled PEO(45)-b-PCL(23) block copolymer was prepared by conjugating a tetramethylrhodamine molecule to the end of the hydrophobic PCL block. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) studies yielded 24 +/- 2 and 25 +/- 2 nm, respectively, for the diameters of the micelles. The studies also showed that chemical labeling did not effect the morphology or size. When the rhodamine-labeled PEO(45)-b-PCL(23) block copolymer micelles were tested in vitro, time-, concentration-, and pH-dependence of the internalization process suggested that internalization proceeded by endocytosis. The results from these studies provide the first direct evidence for the internalization of PEO(45)-b-PCL(23) micelles. Future studies will utilize multiple labeling of these micelles, allowing questions to be addressed related to the fate of internalized micelles as drug carriers, the destination of the incorporated drugs or fluorescent probes released from micelles, and the identification of the subcellular localization of the whole drug-carrier system within cells, both in vitro and in vivo.