Five bacteriophage stocks were prepared after enrichment of a sewage sample using Escherichia coli 02:K1:H4 (strain U9/41). The bacteriophages were tested for their ability to lyse 224 strains of E. coli that had been tested for the presence of the K1 antigen by means of an antiserum-agar diffusion technique, using a meningococcus group B antiserum known to detect the E. coli K1 antigen. The standard test strains for E. coli K antigens 2 to 99 were used as control strains. Of the 101 strains found to possess the K1 antigen using the antiserum-agar technique, 93 were lysed by at least one of the bacteriophages, whereas 8 of the 123 strains apparently lacking K1 were lysed by one or more of the bacteriophages. None of the standard test strains for K antigens 2 to 99 was lysed by any of the bacteriophages. The eight strains thought to lack K1 but that were lysed by bacteriophage were re-examined by immunoelectrophoresis, using meningococcus group B antiserum; five of the eight strains gave a precipitin line corresponding to K1. The use of K1-specific bacteriophages offers an inexpensive and easy method for the identification of the K1 antigen.