Highly purified antibodies to two ubiquitous components of basement membrane, type IV collagen and laminin, were applied to both fresh-frozen and formalin-fixed tissue sections of a variety of invasive carcinomas, carcinomas in situ, and their 'look-alike' benign counterparts. These included lesions of the breast (infiltrating ductal carcinoma, comedocarcinoma, and sclerosing adenosis); lesions of the skin (squamous cell carcinoma, Bowen's disease, and pseudoepitheliomatous hyperplasia); lesions of the pancreas (adenocarcinoma and pancreatitis); lesions of the prostate (adenocarcinoma and benign prostatic hyperplasia); and other epithelial lesions of the invasive, in situ, and benign category. By both immunofluorescence and immunoperoxidase techniques, benign and in situ lesions showed intact basement membranes with linear staining of type IV collagen and laminin. The majority of invasive carcinomas, in contrast, lacked immunoreactivity for both of these basement membrane components. In cases of in situ carcinoma with microinvasion, there was thinning, fragmentation, and disruption of the basement membrane in the foci of microinvasion but not elsewhere. Utilizing antibodies to type IV collagen and laminin aids in both understanding the pathophysiology of the invasive process and the recognition of its presence in tissue sections.