Cellulase can be added to animal feed mixture for the improvement of nutrient conversion. An enzyme‐linked immunosorbent assay has been developed to check the concentration of added enzyme. The IgG fraction from rabbit immune serum against cellulase from Trichoderma viride was labelled with horseradish peroxidase by the glutaraldehyde method. The conjugate (IgG‐Px) was characterized by gel chromato‐graphy on a Superose 12 column. The assay was optimized with respect to the amount of IgG‐Px applied and the reaction conditions. The method developed was utilized for the assay of feed granules supplemented by cellulase preparations. It is possible under the conditions described to determine added cellulase up to a concentration of 24.0 μg/g of the feed mixture.