A collection of hybrid circular DNAs was constructed in vitro using the poly(dA-dT) "connector" method: each hybrid circle contained one molecule of poly(dT)-tailed DNA of plasmid ColE1 (made linear by digestion with EcoRI endonuclease) annealed to a poly(dA)-tailed fragment of yeast (Saccharomyces cerevisiae) DNA, produced originally by shearing total yeast DNA to an average size of 8 X 10(6) daltons. This DNA preparation was used to transform E. coli cells, selecting colicin-E1-resistant clones that contain hybrid ColE1-yeast DNA plasmids. Sufficient numbers of transformant clones were obtained to ensure that the hybrid plasmid population was representative of the entire yeast genome. Various hybrid ColE1-yeast DNA plasmids capable of complementing E. coli auxotrophic mutations were selected from this population. Plasmid pYeleu 10 complements several different point or deletion mutations in the E. coli or S. typhimurium leuB gene (beta-isopropylmalate dehydrogenase); plasmids pYeleu11, pYeleu12, and pYeleu17 are specific suppressors of the leuB6 mutation in E. coli C600. Plasmid pYehis2 complements a deletion in the E. coli hisB gene (imidazole glycerol phosphate dehydratase). Complementation of bacterial mutations by yeast DNA segments does not appear to be a rare phenomenon.