We describe a simple and effective method for inhibition of endogenous peroxidase activity in the immunoperoxidase technique. Specimens are pre-treated with a mixture of azide and hydrogen peroxide, which is then followed by an indirect immunoperoxidase procedure. Comparison studies showed no significant loss of antigenicity or morphological details by this pre-treatment. The method is most useful for evaluating cell-specific antigens on specimens that have abundant endogenous peroxidase activity, such as blood and bone marrow.