Potassium secretion by the kidney is vital for the maintenance of K+ homeostasis. RACTK1, a putative inwardly rectifying potassium channel cloned from cultured rabbit collecting duct cells, has been proposed to play a role in this process. However, the lack of homology with any other cloned potassium channel and the inability to reproduce the results across different laboratories has brought into question the existence of RACTK1. Recently, it has been suggested that RACTK1 is a contamination from Escherichia coli. In this work we add conclusive evidence supporting the bacterial origin of RACTK1. Using both genomic PCR and RT-PCR we were unable to detect RACTK1 in a number of mammalian species. In addition sequencing of RACTK1 cDNA confirmed a complete homology between RACTK1 and a region of E. coli genomic DNA. Finally, a hypothesis on how RACTK1 could have been generated from a contamination by E. coli genomic DNA is presented.