The anti-fluorescein/Oregon Green® mouse monoclonal IgG antibody efficiently binds and quenches the fluorescence of fluorescein and Oregon Green® dyes providing opportunities a means to quench extracellular fluorescence of these fluorophores. Additionally, this antibody enables the use of fluorescein or Oregon Green® dyes as haptens instead of biotin. Unlike biotin, which is an endogenous ligand in mitochondria, dye-based haptens permit background-free staining of cells and tissues and a sensitive alternative to biotin-streptavidin methods for ELISAs. The mouse monoclonal antibody (Clone 4-4-20) has been extensively used in studies focusing on mechanisms of antigen-antibody recognition.