Immunohistochemical alterations in basement membrane components of squamous cell carcinoma. To investigate alterations in the basement membrane (BM) in squamous cell carcinoma (SCC), we investigated 20 tumors. Four had the cytologic characteristics of Bowen's disease (SCC-BD) and 16 did not have them (SCC-NB). Tumors were studied immunohistochemically by double immunofluorescent staining by using mouse monoclonal antibodies to the core protein of heparan sulfate proteoglycan (HSPG) and chondroitin 6-sulfate glycosaminoglycan (Ch6S) as well as rabbit antiserum to laminin (LN) and type IV collagen (C-4). In well-differentiated and highly keratinized SCC-NB, LN, C-4, and HSPG could be detected in the tumor nest BM and showed no loss of continuity, but they were largely lost in poorly differentiated and poorly keratinized SCC-NB. This suggests that poorly differentiated SCC-NB cause greater enzymatic degradation of BM components than well-differentiated SCC-NB. Ch6S was detected in parts of the BM of SCC-BD, but it was absent in all SCC-NB examined. It appears that SCC-NB have lost the ability to synthesize Ch6S, and that SCC-BD degrade Ch6S although they continue to produce it. Thus, it appears that in SCC the BM is qualitatively different from that of normal epidermis, and that SCC-BD can be distinguished from SCC-NB by the Ch6S content of the BM. 