Biotin-X, SE is amino-reactive biotinylating reagent for peptides and proteins; it has a better avidin-binding affinity than biotin. Biotin-X, SE (d-Biotin-amidocaproate N-hydroxysuccinimide ester) contains a six-carbon spacer arm that reduces steric hindrance associated with binding four biotinylated molecules per one avidin and results in enhanced detection sensitivity. The succinimidyl ester (SE) reactive form of biotin-X reacts with the amine group on the protein and forms stable carboxamide bonds, which is identical to natural peptide bonds. Biotin-X-protein conjugates are very stable and can withstand the conditions of immunofluorescent staining, fluorescence in situ hybridization, flow cytometry and other biological applications without hydrolysis. The kit has all essential components for performing the conjugation reaction and for purifying the biotin-protein conjugates. The kit is able to perform three conjugation reactions with each reaction labeling up to 200 µg IgG.