Anti-K1 phages were more active in vitro and in vivo against an O18:K1:H7 ColV+ E. coli strain, designated MW, than were other phages. A single i.m. dose of 1 anti-K1 phage was more effective than multiple i.m. doses of tetracycline, ampicillin, chloramphenicol or trimethoprim plus sulfafurazole in curing mice of a potentially lethal i.m. or intracerebrally induced infection of MW; it was at least as effective as multiple i.m. doses of streptomycin. When MW and the phage were inoculated into different gastrocnemius muscles of the same mice, a rapid reduction in numbers of MW organisms occurred in the MW-inoculated muscle and in other tissues; the numbers of phage particles in the MW-inoculated muscle increased rapidly and greatly. MW failed to proliferate in the brains of intracerebrally infected mice that had been inoculated i.m. with the phage at the same time; many more phage particles were found in the brains of these mice than in other sites. The few phage-resistant mutants of MW found in the phage-treated mice were K1-; previous studies had shown such mutants to be of greatly reduced virulence. The phage administered i.m. 3-5 d [days] before challenge with a potentially lethal i.m. induced infection of MW was protective, the protective effect varying between phage propagated on different bacterial strains.