A methodology for incorporating solubilized CdSe/ZnS core/shell nanocrystals (NCs) into functionalized carboxylated polystyrene latexes 0.3-1 μm in diameter via a swelling procedure was developed and used for the production of homogeneous, highly fluorescent polymeric beads (HFPBs), which were found to be comparable in brightness to standard polymeric microspheres doped with organic fluorophores and more photostable than the latter by more than 50 times (Fluoresbrite yellow-orange microspheres were used as an example). The three-dimensional (3D) confocal analysis of individual 1-μm HFPB demonstrated that the beads were doped with the NCs almost homogeneously. HFPBs 0.3 μm in diameter were conjugated with anti-mouse polyvalent immunoglobulins and used for immunofluorescent detection of p-glycoprotein, a mediator of the multidrug resistance phenotype, overexpressed in the membrane of MCF7r breast adenocarcinoma cells. The photostability of NCs-tagged HFPBs offers obvious advantages for the reconstruction of 3D confocal fluorescence images of antigen distribution, and their exceptionally high brightness combined with photostability permits the detection of a single antigen molecule using a standard epifluorescence microscope. © 2004 Elsevier Inc. All rights reserved.