A series of stepwise polymyxin-resistant envelope mutants of Pseudomonas aeruginosa was used to test the activity of a homologous series (C10-C18) of alkylbenzyldimethylammonium chlorides. A sterilization kinetics procedure in deionized water was devised to avoid amounts quaternary compound above the critical micelle concentration. In all cases, there was a linear relationship between the logarithm of the rate of change of the colony count with time and the logarithm of the homolog concentration. For all strains, there was a linear relationship between alkyl chain length and the concentration required to reduce the colony count to 10% in 2 hr. The stepwise series of polymyxin-resistant strains increased in resistance to polymyxin about threefold for each step. In general, this increase resulted in a similar increase in resistance to the quaternary compound. It is proposed that death in this system may primarily be a consequence of damage to the outer membrane rather than to the cytoplasmic membrane.