The luminal surface of differentiated urothelium is characterised by the presence of asymmetric unit membrane (AUM) which contains four major integral proteins, uroplakins. Cyclophosphamide (CP) causes extensive denudation of the urothelium that is followed by regeneration. In this study the differentiation of the urothelial luminal plasma membrane was examined in CP treated rats by electron microscopy. Single dose of cyclophosphamide was injected and rats were killed after 1, 3, 7, 14 or 28 days. One day after the treatment only few cells remained in the urothelium. They had a flat surface and did not stain with anti-AUM antibody. During regeneration cells with short or pleomorphic microvilli and latter cells with ropy and leafy microridges appeared. Cells with either type of microvilli were not labelled whereas cells with microridges were labelled with anti-AUM. At day 28 the luminal surface was normal and labelled. These results show a good correlation of morphogenesis and the expression of uroplakins.