Hyperplastic and metaplastic lesions in the prostate gland of bulls are used as morphological evidence for the illegal administration of oestrogens or compounds with oestrogenic activity, like stilbenes. Exogenous and endogenous androgens can suppress the effects of these drugs, resulting in suspect or negative results. The detection of epithelial changes immunohistochemically with cytokeratin antibodies appeared to facilitate the diagnosis of hyperplasia and metaplasia in the prostate. A specific and sensitive method for detecting such lesions in formalin-fixed, paraffin-embedded prostate samples has been developed with the commercially available cytokeratin monoclonal antibody 34βE12, and applied to bulls treated with diethylstilbestrol-dipropionate. The highly reproducible staining pattern of 34βE12 can be used for the detection of hyperplasia and metaplasia in the prostate glands of animals treated illegally with oestrogens.