The presence or absence of basement membrane (BM) was examined in normal and neoplastic adult prostatic tissue as well as prostate cell lines using immunohistochemistry and electron microscopy. Immunohistochemical studies using antibodies directed against laminin and type IV collagen were done on 55 samples of human prostate representing various grades of prostate carcinoma. The percentage of glandular structures surrounded by BM was determined. Benign prostates (n = 15) had BM around 99% of the acini. Gleason Grade II adenocarcinoma (n = 9) had 65%, Grade III (n = 5) had 23%, Grade IV (n = 12) had 15%, and Grade V (n = 7) had 0% BM around glandular structures, respectively. None of the metastases (n = 7) had visible BM. By transmission electron microscopy, 32 prostates were examined for the glandular profile of BM. One hundred percent of the acini in the benign prostates (n = 17) had BM. Of the low grade carcinomas, Gleason I and II (n = 4), 44% of the acini had BM, and of the high grade carcinomas, Gleason IV and V (n = 7), 34% had BM. None of the metastases (n = 4) had BM by electron microscopy. We conclude that, in prostatic carcinoma, there is a progressive loss of BM with decreasing differentiation, and that in prostate carcinoma metastases there is a complete loss of epithelial BM.