Although the elaboration of bacteriocins by streptococci has been described previously (Brock & David, 1963; Kuttner, 1966; Kelstrup & Gibbons, 1969; Overturf & Mortimer, 1970; Kramer & Brandis, 1972), few attempts have been made to purify and characterize these bactericidal factors (Kramer & Brandis, 1972). Streptococcus sanguis (strain Challis) produces a streptocin which is lethal for Streptococcus sanguis (strain Wicky) and can be purified by ammonium sulphate fractionation and Sephadex G-100 column chromatography. We exposed sensitive strain Wicky cells to Challis streptocin and observed macromolecular synthesis and membrane permeability.