Gap junctions enable intercellular communication and play an important role in a variety of vital cellular functions including differentiation and the control of growth. These junctions are formed by a hexameric of proteins known as connexins. We investigated the distribution of the connexin 43 (Cx43) gap junction protein in renal cells and human kidney using the alkaline phosphatase anti-alkaline phosphatase immunohistochemical technique with a monoclonal antibody directed against the cytoplasmic domain of this antigen. Strong staining was demonstrated on the vascular endothelium, the smooth muscle of larger vessels and on glomerular epithelial cells. In addition, Cx43 was expressed on proximal tubular cells, glomerular endothelial cells and occasional cells infiltrating the interstitium. In areas of tubular atrophy there was increased staining for Cx43. Using reverse transcription-polymerase chain reaction we have also demonstrated that cultured human and rat mesangial cells and human proximal tubular cells express Cx43 messenger RNA. In summary, we have described for the first time the distribution of Cx43 in human kidney and cultured renal cells.