Thioglycolic-acid-stabilized CdTe quantum dots, synthesized directly in aqueous solution, are successfully conjugated with biotin and polyethylene glycol. Using these conjugates, we report the development of this kind of water-soluble quantum dot for immunofluorescent labeling of cancer cells. The results show that these conjugates have very low nonspecific binding and good stability against photobleaching, enabling them to be applied in many biological fields, such as cellular labeling, intracellular tracking, and other imaging applications.