A methodology for simple convenient preparation of bright, negatively or positively charged, water-soluble CdSe/ZnS core/shell nanocrystals (NCs) and their stabilization in aqueous solution is described. Single NCs can be detected using a standard epifluorescent microscope, ensuring a detection limit of one molecule coupled with an NC. NCs solubilized in water by DL-Cys were stabilized, to avoid aggregation, by poly(allylamine) and conjugated with polyclonal anti-mouse antibodies (Abs). NC-Abs conjugates were tested in dot-blots and exhibited retention of binding capacity within several nanograms of antigen detected. We further demonstrated the advantages of NC-Abs conjugates in the immunofluorescent detection and three-dimensional (3D) confocal analysis of p-glycoprotein (p-gp), one of the main mediators of the MDR phenotype, overexpressed in the membrane of MCF7r breast adenocarcinoma cells. Immunolabeling of p-gp with NC-Abs conjugates was 4200-, 2600-, and 420-fold more resistant to photobleaching than its labeling with fluorescein isothiocyanate-Abs, R-phycoerythrin-Abs, and AlexaFluor488-Abs, respectively. The labeling of p-gp with NC-Abs conjugates was highly specific, and the data were used for confocal reconstruction of 3D images of the p-gp distribution in the MCF7r cell membrane. Finally, we demonstrated the applicability of NC-Abs conjugates obtained by the method described to specific detection of antigens in paraffin-embedded formaldehyde-fixed cancer tissue specimens, using immunostaining of cytokeratin in skin basal carcinoma as an example. We conclude that the NC-Abs conjugates may serve as easy-to-do, highly sensitive, photostable labels for immunofluorescent analysis, immunohistochemical detection, and 3D confocal studies of membrane proteins and cells. © 2003 Elsevier Inc. All rights reserved.