17β-Oestradiol is a steroid hormone banned as a growth promoter in food-producing animals all over Europe because of its carcinogenicity. Despite mandatory monitoring of illegal treatment all over Europe, official analytical methods in use test negative a few days after 17β-oestradiol administration, requiring new sensitive tools to ensure a high level of protection for consumers. The aim of this work was the evaluation of the accuracy of histopathology and immunohistochemistry for progesterone receptor (PR) as a screening method for the detection of low-dosage illegal treatments with 17β-oestradiol. Fresian male calves (153) were farmed under controlled conditions, and 89 of them were treated with 17β-oestradiol (5 mg/animal once a week for 4 weeks). After 15 days of suspension, all animals were slaughtered and sexual accessory glands (prostate and bulbo-urethral glands) were sampled for histological examination and immunohistochemical staining with anti-PR antibody (clone hPRa 2). Microscopically 86 out of 89 bulbo-urethral glands showed mild to severe metaplasia, while mild metaplasia was observed only in 1 control. Eighteen out of 89 samples of prostate did not show metaplastic lesions. Immunopositivity for PR characterised all treated animals, while no signal was detected in controls. These findings show that metaplasia of the sexual accessory glands is a sensitive and specific parameter for illegal 17β-oestradiol treatment in calves at the slaughterhouse, while the appliance of immunohistochemistry for PR can improve to 100% the accuracy of this highly reliable histological approach.