The antibacterial activity of the association of ampicillin and sulbactam (2:1) upon strains of aerobic and anaerobic Gram negative agents was assayed. Inhibition of B-lactamases from intact and disrupted cells of some selected strains by sulbactam was also determined. Susceptibility was found in 34% and partial inhibition in 48%; 18% were fully resistant. Synergy between ampicillin and sulbactam was found upon ampicillin susceptible strains. This effect might reflect higher degree of binding of the inhibitor to PBP's in the absence of a trapping effect produced by B-lactamases. Anaerobic microorganisms were found to be highly susceptible to this association of B-lactams. Adequate penetration of the inhibitor into periplasmic space of Gram negative bacilli was deduced by the similar decrease of B-lactamase activity observed in intact and disrupted cells treated with sulbactam.