Immunoperoxidase staining method and specific antisera against highly purified inhibin-like material (ILM) from human seminal plasma were utilized to localize immunoreactive ILM in benign hyperplastic prostate (BPH) in human. Immunoreactive ILM was strictly localized to glands of hyperplastic nodules. The concentration of ILM was more in epithelial cells lining the healthy lobule than in those undergoing atrophy.