The problem of the origin in vitro of a particular type of cell population (monolayers of epitheliocytes) from mouse kidney was investigated by applying neotetra-zolium and trypan blue as extrinsic tags to the donor animals. No signs of the extrinsic tags were found in the monolayers in any of the experiments, though other tubular elements of the explants did reveal formazan or trypan blue inclusions when the donor animals were treated with neotetrazolium or trypan blue respectively. In the intact kidney of the treated animals the tags were found in the cells of the proximal tubules. It therefore can be safely concluded that the epithelial monolayers of the explants do not originate from cells of the proximal tubules. However, their origin from other epithelial elements of the kidney (collecting tubules?) cannot be excluded.