AbstractImprovements in the reliability of immunoperoxidase techniques have resulted in an increase in their use in diagnosing human disease. A large number of antibodies against human antigens are available commercially, which has expanded the use and application of these techniques in human pathology. Veterinary pathology has the same requirements for diagnosing diseases encountered in other species. With that need in mind, an assessment of the cross-species reactivity of antibodies to human antigens using an immunoperoxidase technique was conducted. The majority of the antigens selected for testing in the two nonhuman species showed a strong reactivity to the antihuman antibodies. The results and the recommendations for obtaining optimum results are reported. (The J Histotechnol13:255, 1990)