The activity of cefuroxime, ciprofloxacin and imipenem against 50 coagulase-negative staphylococci (CNS) strains isolated from patients with peritonitis while on continuous ambulatory peritoneal dialysis (CAPD) was examined. Strains were cultured in both used peritoneal dialysis fluid (PUD) and broth (ISB) in an atmosphere of air with 5% CO2. MICs of cefuroxime, ciprofloxacin and imipenem differed significantly between the two media for 40%, 26% and 42% of strains, respectively. In three-quarters of these cases the MICs were higher in PUD compared to ISB. The ability of each antibiotic to kill six strains of CNS adherent to silicone rubber was also examined. Once again, differences in killing were noted between the two media. At least two-thirds of the strains remained viable in the presence of concentrations of antibiotic equal to the MBC of each antibiotic and at least one-third survived antibiotic concentrations equivalent to those attained during treatment. Assessment of activity in PUD may provide a more realistic test for antibiotics to be used in the treatment of CAPD peritonitis.