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SUMMARYPurified lipopolysaccharide (LPS) from the bacteriocin sensitive strain Rhizobium lupini 16-2 was shown to neutralize the killing activity of the bacteriocin. In the electron microscopical preparation the phage tail-like bacteriocin appears to be adsorbed to the LPS; the tail sheath is contracted and the fibres are oriented towards the LPS ribbon. In contrast, no interaction was observed between the bacteriocin and the LPS of two resistant strains of . The inactivation of the bacteriocin by LPS depends on salt concentration, pH, and temperature. The receptor activity of LPS was destroyed by mild acid hydrolysis and by treatment with deoxycholate, which indicates that the micellar structure of the LPS is necessary for bacteriocin adsorption. The chemical composition of the 16-2 LPS was compared to that of the LPS of two resistant strains. In the case of i6-2/I1 LPS minor modifications suffice to confer resistance against the bacteriocin.