The use of m-chloroperoxybenzoic acid (mCPBA) in stead of hydrogen peroxide causes an increase in chemiluminescence (CL) of luminol oxidation catalyzed by microperoxidase 8 (MP8) by an order of magnitude. The accelerated formation of an intermediate plays a major role in the CL enhancement, which also leads to a significant reduction in CL duration. The presence of guanidine hydrochloride, sodium carbonate, or sodium chloride further increases the CL emission drastically. The CL emission enhancement is strongly pH dependent. The enormous enhancement of the CL signal is due to an accelerated CL cycle and an improved CL efficiency in the presence of the enhancer. The CL signal covers several orders of magnitude over a wide range of concentrations of luminol and mCPBA. The intense CL of MP8-luminol-mCPBA in the presence of the enhancer will have great potential for extremely sensitive CL assays.