Selection and regrowth of resistant variants, which were present in low frequencies in the initial inoculum, was seen when large inocula of Pseudomonas aeruginosa were incubated with imipenem. The selective growth of resistant variants resulted in an inoculum effect, and an increase in MIC with longer incubation. When large inocula taken from strains that had been classified as sensitive in conventional MIC determinations were incubated with 4, 8 and 16 mg/l imipenem, 62%, 24% and 10%, respectively, of the strains regrew. None of the resistant variants thereby selected showed cross resistance to other beta-lactam antibiotics or aminoglycosides. In vitro evaluation of regrowth of resistant variants may be justified when choosing imipenem for therapy in the treatment of serious P. aeruginosa infections.