Small pieces of tissue obtained from apparently normal areas of four surgically removed adult human kidneys were used in the present study. The results obtained by immuno-fluorescence and immuno-electron microscopical techniques show that Tamm-Horsfall glycoprotein (THP) is present in the thick ascending limbs of the loops of Henle and the distal convoluted tubules. Within the cells concerned, the protein is associated with the luminal, lateral as well as basal, plasma membranes and their infoldings. The cells of the macula densa are completely negative as are those of proximal convoluted tubules, glomeruli and collecting ducts. The possible significance of these findings in relation to the process of urine dilution in the nephron is discussed.