Semiconductor quantum dots (QDs) offer several advantages over organic dyes in fluorescence-imaging applications, such as higher quantum yield, exceptional photostability, and a narrow, tunable, and symmetric emission spectrum. To explore whether QDs could specifically and effectively label tumor markers and be used in immunohistochemistry as a novel type of fluorescent probe, we used quantum dots with maximum emission wavelength 605 nm (QD605) to detect the ovarian carcinoma marker CA125 in specimens of different types (fixed cells, tissue sections, and xenograft piece). Additionally, we compared the photostability of QD signals with that of a conventional organic dye, FITC. All labeling signals of QDs were found to be more specific and brighter than those of FITC. Moreover, the QDs exhibited exceptional photostability during continuous illumination for 1 h by a high-intensity laser (Ar laser power 100 mW) at 488 nm, while the FITC signals faded very quickly and became undetectable after 24 min of illumination. These results indicate that QD-based probes can offer substantial advantages over existing fluorophores in many applications, and can be used effectively in immunohistochemistry as a novel class of fluorescent probes.