In order to assess the expression of different cytokeratins in the collecting duct cells (CDCs) of the human kidney, three consecutive sections were stained with periodic acid-Schiff, CAM 5.2 and AE-1 (CAM 5.2 recognizes cytokeratins #19,18,8 and AE-1 #19,16,15,14,10 of Moll's catalog.), respectively. By comparing these sections, it was found that most CDCs in the inner medulla were both CAM 5.2- and AE-1-positive, whereas in the outer medulla and cortex, 77% of the CDCs were both CAM 5.2- and AE-1-positive, 15% CAM 5.2-positive and AE-1-negative, 8% both CAM 5.2- and AE-1-negative, and 0.4% CAM 5.2-negative and AE-1-positive. Recent studies have shown that most CDCs express low-molecular-weight cytokeratins #7,8,18 and 19 (17, 18, 19, 20). Of these cytokeratins, CAM 5.2 recognizes cytokeratins #8,18,19 and AE-1 recognizes cytokeratin #19. Therefore, most CDCs belong to one of the following three major types; 1. Those positive for cytokeratins #8,18 and 19 (CAM 5.2- and AE-1-positive), 2. Those positive for cytokeratins #8 and 18 and negative for #19 (CAM 5.2-positive and AE-1-negative) and 3. Those negative for cytokeratins #8,18 and 19 (CAM 5.2- and AE-1-negative). A few CAM 5.2-negative and AE-1-positive cells were thought to express high-molecular-weight cytokeratins. The significance of these various cytokeratin expressions is discussed.