The effect of the age of biofilms formed by Acinetobacter baumannii on the activity of sulbactam and imipenem on sessile cells was investigated. Against cells of young biofilms both antibiotics were rapidly bactericidal. As the biofilm aged, sulbactam lost most of its activity, but imipenem continued to be bactericidal to some extent. This fact probably reflects the ability of imipenem to exert bactericidal activity on slow-growing bacteria.