The basal lamina in a variety of metastatic tumours in brain was assessed with M antibody to type-IV collagen and the indirect immunoperoxidase technique. The antibody was raised in rabbits against type-IV collagen isolated from human placental tissue. Basal lamina redevelopment was demonstrated around individual melanoma cells, between melanoma cells and cerebral parenchyma, and at the tumour-stroma interface in both metastatic melanoma and metastatic carcinoma. At the periphery of metastatic carcinomatous deposits, no basal lamina was observed between tumour cells and the adjacent cerebral parenchyma. Basal lamina staining other than that of cerebral vessels was absent in deposits of poorly differentiated tumours which were unaccompanied by the development of a tumour stroma It is concluded that metastatic tumours retain the ability to produce basal laminaand, in the case of metastatic epithelial turnours, the redevelopment of basal lamina is dependent on interaction with mesenchymal tissue. The strornal dependency of basal lamina formation by metastatic epithelial tumours suggests the reactivation of a control mechanism acting in normal tis- sue. Although basal lamina formation in metastatic melanoma can occur in the absence of mesenchymal tissue, there may be some interaction between tumour cells and stroma in the redevelopment of basal lamina at the tumour-stroma interface.