Anti-K 1 phages were more active in vitro and in vivo against an 018:K1:H7 ColV+ Escherichia coli strain, designated MW, than were other phages. A single intramuscular dose of one anti-K1 phage was more effective than multiple intramuscular does of tetracycline, ampicillin, chloramphenicol, or trimethoprim plus sulphafurazole in curing mice of a potentially lethal intramuscularly or intracerebrally induced infection of MW; it was at least as effective as multiple intramuscular 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 intramuscularly 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 mich were K1-; previous studies had shown such mutants to be of greatly reduced virulence. The phage administered intramuscularly 3-5 d before challenge with a potentially lethal intramuscularly induced infection of MW was protective, the protective effect varying between phage propagated on different bacterial strains.