Resistance and the development thereof in Pseudomonas aeruginosa to the bactericide sodium dimethyldithiocarbamate (SMT) was investigated. P. aeruginosa was cultured in nutrient-poor broth in the presence of subinhibitory concentrations of SMT. It adapted over 21 days of exposure from 250 mug . ml-1 to 490 mug . ml-1. The initial high MIC was ascribed to exclusion of SMT by the lipopolysaccharide layer, since removal thereof by EDTA rendered cells highly susceptible. The alginate-producing mutant PAO 579 was much more susceptible to SMT than was its parent PAO 381, indicating that extracellular polysaccharide does not act as an exclusion barrier to SMT. Following 24 h of exposure to SMT, P. aeruginosa had an altered profile of outer membrane proteins as determined by SDS-PAGE. Resistant cells had a further altered profile. Resistance of P. aeruginosa is ascribed to a change in the outer membrane protein profile, leading to improved exclusion of SMT.