The Biotin-SP (long spacer)-conjugated protein listed here is offered for the indirect bridged avidin-biotin (IBRAB) method of Hsu et al. (J. Histochem. Cytochem. 1981. 29, 577). However, despite the possible advantages of this method, it may not be as sensitive as other biotin- and streptavidin-based methods (J. Histochem. Cytochem. 1989. 37, 1609; J. Histochem. Cytochem. 1992. 40, 135).