Staphylococcus aureus strain 26 inhibited the growth of 23 of 26 lactobacilli of endocervical origin, but only two of 17 staphylococci, in deferred antagonism tests. The inhibitory agent, a bacteriocin-like inhibitory substance (BLIS) named staphylococcin Au-26, was obtained from vigorously shaken liquid cultures containing a 0.1% (v/v) supplement of Tween 80 and was purified by chromatographic fractionation on XAD-2, carboxymethyl Sephadex and reversed phase HPLC. The molecular mass of staphylococcin Au-26 was estimated by SDS-PAGE to be approx. 2700. The detection of lanthionine residues in the molecule, the high stability to heating at acidic but not alkaline pH values and inactivation by proteinases indicate that staphylococcin Au-26 is a member of the lantibiotic class of peptide antibiotics--the first reported to be produced by a S. aureus strain. Primary sequence analysis showed that the N-terminus of the molecule is isoleucine, a characteristic also displayed by the lantibiotics nisin, epidermin and gallidermin.