Some species of bacteria have low-permeability membrane barriers and are thereby "intrinsically" resistant to many antibiotics; they are selected out in the multitude of antibiotics present in the hospital environment and thus cause many hospital-acquired infections. Some strains of originally antibiotic-susceptible species may also acquire resistance through decreases in the permeability of membrane barriers. Another mechanism for preventing access of drugs to targets is the membrane-associated energy-driven efflux, which plays a major role in drug resistance, especially in combination with the permeation barrier. Recent results indicate the existence of bacterial efflux-systems of extremely broad substrate specificity, in many ways reminiscent of the multidrug resistance pump of mammalian cells. One such system seems to play a major role in the intrinsic resistance of Pseudomonas aeruginosa, a common opportunistic pathogen. As the pharmaceutical industry succeeds in producing agents that can overcome specific mechanisms of bacterial resistance, less specific resistance mechanisms such as permeability barriers and multidrug active efflux may become increasingly significant in the clinical setting.