CD46 is a cell-surface regulatory molecule that prevents lysis of autologous human cells by activated complement. It has been well characterized on leucocytes, reproductive cells and various cultured cell lines and is considered to be ubiquitously expressed. We now extend these analyses and describe CD46 in a variety of different human tissues. Strong expression was observed by immunohistology on epithelial cells lining exocrine ducts and glands, such as salivary gland and pancreas and on kidney tubules and glomerular epithelium. Quantitative tissue expression was measured by radioimmunoassay and confirmed histological observations. Thus, CD46 is highly expressed on cells in contact with extracellular fluids thought not to contain large quantities of complement but which may still be subjected to complement attack thereby necessitating the presence of complement regulators to prevent non-specific destruction of cells.