DNP-X-biocytin-X, SE reacts efficiently and reproducibly with amines. The biotin is well separated from the point of attachment by a spacer consisting of a seven-atom aminohexanoyl group (X) and a seven-atom lysine residue. This spacer enhances the accessibility of biotin to the relatively deep biotin-binding sites of avidin and streptavidin. Because the molecule is labeled simultaneously with biotin and DNP, the degree of biotinylation can be assessed from the optical absorbance of the DNP moiety.