Twelve 4-quinolones (cinoxacin, ciprofloxacin, enoxacin, flumequin, nalidixic acid, norfloxacin, oxolinic acid, pefloxacin, pipemidic acid, rosoxacin, and piromidic and beta-hydroxypiromidic acids) and novobiocin, were used at subinhibitory concentrations to eliminate from Escherichia coli 11 antibiotic resistance plasmids belonging to different incompatibility groups. The 12 4-quinolones were also tested for their ability to cure virulence plasmids from five species of Enterobacteriaceae. All quinolones eliminated three antibiotic resistance plasmids (R446b, R386, S-a) and one virulence plasmid (pWR105), but at a low rate. Optimal curing of antibiotic resistance plasmids was obtained in human urine. Two virulence plasmids (pWR24 and pWR110) were eliminated only by flumequin and pefloxacin. Novobiocin eliminated three antibiotic resistance plasmids (R446b, R386, pIP24). The variable and low level of plasmid loss may be explained by the induction of the recA system. In addition, the inability to eliminate certain plasmids could be due to their presence in high numbers per cell.