Coagulase-negative staphylococci (CNS) are common causes of infection in patients undergoing chronic ambulatory peritoneal dialysis (CAPD). Their ability to survive intracellularly within peritoneal macrophages and to persist within the peritoneum during antibiotic therapy has led to the development of drug resistance during treatment. Strains of Staphylococcus epidermidis (SE) and Staphytococcus haemolyticus (SH) have been isolated from patients with CAPD during treatment with ciprofloxacin. The respective MIC values pre-and post-therapy were SE-0.25 and 128 mg/L and SH-0.50 and 64 mg/L. The susceptibility of each isolate to opsonophagocytosis was measured in vitro using isolated polymorphonuclear leucocytes (PMN) derived from fresh human blood donations. The bacteria were radiolabelled during growth, opsonised in either 1 or 10% serum and their uptake measured No differences were seen between the pre- and post therapy isolates when using 10% serum as opsonic source (18 vs. 21%); with 1% serum the corresponding values were lower (5 and 8% respectively). Similarly their ability to generate a respiratory burst as measured by chemiluminescence (CL) in the phagocytic cells was not diminished in the strains which had developed resistance to ciprofloxacin. The mean CL response to the strains isolated at outset of therapy ranged from 0.35-0.45 cpsc, and to the resistant strains following therapy from 0.36-0.50 cpsc. It is clear from the present investigation that although the bacterial strain became at least 10 times more resistant to ciprofloxacin during therapy, no change in their susceptibility to phagocytosis occurred refuting the idea that the emergence of drug resistant strains during therapy results in "super-bugs" of greater virulence.