Abstract
When E. coli K235 is grown in the presence of mitomycin C it elaborates part of its colicin as a protein, which has been purified by gel filtration and electrofocusing to yield a product that is serologically homogeneous. The colicin K of E. coli K235 is chemically and serologically identical with that of Proteus mirabilis. Like the colicin of the latter, the E. coli bacteriocin occurs in multiple forms that exhibit slight differences in mobility upon electrophoresis in a polyacrylamide gel.