Once prepared, the monoclonal antibody to Bromo-deoxyuridine reagent contains a nuclease to produce single stranded DNA which is easily accessible to the antibody without the need for chemical denaturation. This enables detection of cell proliferation while maintaining excellent cell morphology. The improved preservation of antigens also allows double immunocytochemical staining studies to be carried out. The antibody/nuclease mixture may be used for the study of cell proliferation when a choice of detection system is required. This reagent is compatible with the large range of detection systems available from Cytiva, including fluorescent labelled antibodies and Biotin-Streptavidin detection reagents.