Acridinium biotin is a conjugate of acridinium ester and biotin. It is a chemiluminescent label that can be used to detect antibodies or antigens in a wide range of immunoassays. For example, it may be used to label antibodies or antigens that can be detected by streptavidin-coated magnetic beads in a chemiluminescent immunoassay system. Since biotin is a small molecule that can bind to streptavidin with high affinity and specificity, the acridinium biotin reagent can be used in a variety of immunological applications in combination of numerous commercially available streptavidin conjugates.