The ultrastructure and histochemistry of the human urethral mucosa was studied. By scanning electronmicroscopy (SEM) the cover cell of the urethra was found to be polygonal and with a surface topography characterized by numerous microvilli and micro-ridges. By transmission electronmicroscopy (TEM) the cover cells were shown to be interconnected with tight junctions but to lack the asymmetric luminal membrane and the fusiform vacuoles that characterize urothelium above the bladder neck. Histochemical analyses showed the human urethral cells to harbour large amounts of glycogen, and the glycocalyx facing the urethral lumen displayed high affinity for alcian blue and colloidal iron, indicating the presence of acid mucopolysaccharides. The reactivity with alcian blue appeared at pH 2.5, but was abolished by pre-treatment with sialidase. Studies with TEM or using SEM with energy-dispersive X-ray spectrometry (EDX) confirmed the high content of acid mucopolysaccharides in the luminal glycocalyx of the cover cells by demonstrating high binding capacity for ruthenium red. The quantitative binding of ruthenium red was not influenced by pH shifts between 4.5 and 7.5. Utilizing the SEM + EDX technique, small variations in quantity of negative charge (i.e. of bound ruthenium red) were detected within individual cover cells, but considerable variations were found between cells. The significance of these physicochemical properties of the human urethral lining is discussed with special reference to the previously demonstrated liability of this mucosal surface to interact with microorganisms such as Escherichia coli, Staphylococcus saprophyticus and Neisseria gonorrhoeae.