The changes in the basement membrane were studied in human transitional cell carcinoma of the bladder by ordinary transmission electron microscopy and through histochemical analysis of laminin (a basement membrane specific glycoprotein). In normal transitional epithelium, the basement membrane is composed of two distinct layers, i.e., an electron-lucid layer near the cell surface, called the lamina lucida, and a denser layer close to the connective tissue, called the lamina densa. Laminin was localized in the lamina lucida in contact with the basal cells. In carcinomas these structures gradually change, and this was quite obvious in an invasive one. However, the localization pattern of laminin was constant. In other words, the lamina densa changed in various ways: rupture, detachment from cells, multilayering, and thickening, etc., but laminin was always situated around carcinoma cells and separated them from the surrounding tissue. Although the meaning of these findings is not clear now, it seems that, in the light of its biological activity previously reported, laminin may be produced by carcinoma cells in their proliferation process and may assist the invasion and metastasis of carcinoma cells.