We have discovered that the stability of a dilute peroxidase-containing solution is greatly enhanced by the addition of a specific substrate for the peroxidase, in the absence of peroxide. We provide a peroxidase-containing reagent consisting of a buffered aqueous solution comprising a peroxidase or a peroxidase conjugate and a specific substrate for the peroxidase, in the absence of peroxide. The peroxidase may be a free peroxidase but for assay purposes is preferably a peroxidase bound to a specific binding component of an assay, to form a peroxidase conjugate. The peroxidase conjugate is preferably a conjugate between a peroxidase and an antigen or an antibody, most preferably an antibody. The peroxidase is preferably horseradish peroxidase.