The distribution of the various basement membrane (BM) components (type IV collagen, laminin and heparan sulphate proteoglycan) was studied in fetal, adult normal, hyperplastic and neoplastic prostates in formalin- and ethanol-fixed paraffin-embedded specimens. Stromal, epithelial and neoplastic BMs expressed differential susceptibility to pepsin treatment, suggesting conformational differences in the expression of epitopes on BM proteins in distinct anatomical structures and various lesions of the human prostate. In fetal prostate the acinar BM was regular and continuous in contrast to normal adult prostate and various hyperplastic conditions where the acinar BM was locally thickened or unreactive to the anti-BM antibodies. The localization pattern of BM components in grade I and grade II phases of prostatic cancer did not differ essentially from those found in various hyperplastic lesions. Regardless of the histopathological grade of malignancy, prostatic carcinoma cells were surrounded by distinct pericellular and periacinar membranes which were present even at points of contact with the stroma. This suggests that stroma invasion is invariably associated with neoplastic BM formations. Immunohistochemical evidence of the stromal or epithelial origin of neoplastic BMs could not be found. However, the consistent extracellular distribution of neoplastic BM components in contact with the stroma indicates that the elaboration of BM material requires a stromal influence.