The recent determination of 3D X-ray crystal structures for the prokaryotic inwardly-rectifying Kir channels KirBac1.1 and KirBac3.1 has provided a major advance in our understanding of the relationship between structure and function for this class of potassium channels. However, very little is known about the functional properties of these prokaryotic Kir channel homologs. In this study we have cloned KirBac genes from a diverse range of prokaryotic organisms and unicellular eukaryots. These 10 different KirBac sequences exhibit between 24% and 67% identity with KirBac1.1 at the amino acid level and show sequence diversity in structurally important regions. In order to assess their functional properties we have screened their ability to complement the growth of mutant E.coli strains deficient in K+ uptake (TK2420 and LB2003). Several of the KirBac gene products, including KirBac1.1 appear toxic to the sustained growth of E.coli and so appropriate host-independent expression vectors must be used. For this purpose we constructed our own expression vector based upon the Qiagen pQE-60. Complementation of growth was assessed using both standard and defined low K+ medium in culture and on plates. Some KirBac channels e.g. KirBac2.1 and KirBac6.1 appear to show highly efficient complementation of growth at 10mM K+. The KirBac channels also appear to exhibit differing sensitivities to inhibition by external barium and caesium. In particular growth of strains containing KirBac6.1 could be completely inhibited by 10 micromolar caesium chloride whilst other channels e.g. KirBac1.1 and KirBac2.1 require millimolar concentrations to achieve the same effect. These results demonstrate a clear range of functional diversity amongst this family of KirBac channels which, combined with the high resultion structural information now available, should provide an excellent resource for future structure-function and physiological studies.