Antisera to the basement membrane proteins laminin, fibronectin and type IV collagen were prepared, characterized and rendered monospecific by appropriate treatments. Methods were developed to allow the use of these antisera for inmunohistochemical staining of sections of tissue which had been preserved by several methods. In particular, the use of protease digestion {"unmasking") to allow staining of formalin-fixed, paraffin-embedded material was studied. The presence and intensity of staining was found to be highly dependent on the protease and the conditions of digestion, the type of basement membrane, and whether the tissue was normal or neoplastic. The distributions of the three proteins were studied in normal colorectal mucosa and in colorectal adenocarcinoma. A detailed retrospective study was made of the distribution of laminin in 158 cases of rectal carcinoma. Tumours fell into two groups: those which showed linear basement membrane-like laminin staining (laminin positive) and those which did not (laminin negative). Patients with laminin positive tumours had a reduced incidence of distant metastasis and an increased 5 yr survival rate; these correlations were statistically highly significant. Carparison by multivariate analysis with other widely used prognostic markers indicated that laminin status has considerable potential for use as a prognostic marker in the management of such patients. The antisera were also used in a study of the cellular origin and biosynthesis of basement membrane proteins in three systems. In a basement membrane-producing murine tumour, intracellular staining was seen, but it was found that different methods of tissue preparation and unmasking drastically affected the apparent distributions of the three antigens. In the developing rat intestine, no evidence was seen of basement membrane synthesis by the intestinal epithelial cells. However in isolated rat intestinal epithelial cells, some evidence was found for synthesis of laminin and fibronectin, but not type IV collagen.