Immunohistochemical detection of rat CAR bacillus antigen in paraffin-embedded experimentally infected rat lungs, using an immunoperoxidase technique based on the labelled streptavidin biotin (LSAB) method and 3-amino-9-ethylcarbazole (AEC) as substrate is described in this paper. The pattern of immunostaining was confined to the ciliated bronchial epithelium and the specificity of this technique was confirmed. The use of AEC as substrate was evaluated more efficient than diaminobenzidine (DAB). The usefulness of this immunoperoxidase technique for the detection of CAR bacillus in rats and its advantages compared to the indirect immunofluorescence (IF) are discussed.