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Present luminescence assays for horseradish peroxidase (HRP) have limitations. Here we report a novel procedure in which the HRP-catalysed luminescence of a cyclic hydrazide (such as luminol) is multiplied severalfold by the addition of a synthetic component of the firefly bioluminescent system, D-luciferin (4,5-dihydro-2-(6-hydroxy-2-benzothiazolyl)-4-thiazole-carboxylic acid). The specific enhancement of HRP-catalysed light emission from cyclic hydrazides should extend the sensitivities of luminescently monitored assays, which have already been shown to be as sensitive as those using radioactive labels^1,2. This procedure has been applied to the immunoassay of serum α-fetoprotein, thyroxine, digoxin, hepatitis B surface antigen, immunoglobulin E and rubella virus antibody.