The objective of this study was to produce and assess novel vaccines for the protection of broiler chickens against respiratory and septicemic infection caused by 'Escherichia coli'. In the first part of the study, the adhesins of type 1 fimbriae (FimH) and P fimbriae (PapG), the aerobactin receptor protein (IutA), and lipopolysaccharide (LPS) of a virulent avian 'E. coli' (strain EC99 of O78 serogroup) were evaluated as vaccine candidates. The protein antigens were purified as hexahistidine-tagged recombinant proteins, and LPS was purified from a phenol/water extract. Initially, antibodies to these antigens were shown to be produced by experimentally infected chickens. Subsequently, antibodies extracted from the yolks of eggs laid by hens vaccinated with the antigens were used to determine the protective effect of anti-FimH, anti-PapG, anti-IutA, and anti-LPS antibodies. Antibodies against live EC99 'E. coli' and killed EC99 'E. coli' were assessed at the same time. Chickens were given 100 mg of purified IgY intramuscularly at 11 days of age, and were challenged 3 days later with homologous (O78) or heterologous 'E. coli' (O1 and O2) via the intra-air sac route. All the antibodies except anti-FimH protected the chickens from the homologous 'E. coli'. Anti-PapG and anti-IutA antibodies were also effective against heterologous challenges. The protection conferred by anti-PapG antibodies against heterologous challenge was greater than that provided by anti-IutA (85% vs 72.5%). In the second part of the study, ' galE', 'purA', and 'aroA' mutants of ' E. coli' EC99 were constructed by allelic exchange mutagenesis and evaluated as live vaccines. The mutants showed the expected phenotypic and genotypic characteristics, did not revert, and were attenuated for virulence in 1-day-old chicks. Spray vaccination of chickens at 7 and 14 days of age revealed that the mutants were safe and induced moderate levels of antibodies against whole 'E. coli'. Efficacy of the mutants as vaccines was assessed in an experimental infection involving administration of intranasal infectious bronchitis virus followed by an 'E. coli' aerosol. The three mutants were effective against homologous ('E. coli' O78) but not against heterologous challenge ('E. coli' O2).