the preferable route for treatment of peritoneal dialysis related peritonitis remains the intraperitoneal administration of antibiotics admixed to peritoneal dialysis fluids. It is important to know whether the administered drug is compatible with the PD fluids and its container. In the present study the compatibility of aztreonam with four commercial PDFs at storing temperatures and duration representative for storing conditions in the clinical settings was investigated. Aztreonam concentrations were determined using high-performance liquid chromatography. The antimicrobial activity of aztreonam was evaluated using an E. coli diffusion disk inhibition assay and P. aeruginosa time-kill curves. In Extraneal evaluated at 6 °C, 25 °C and 37 °C aztreonam was stable over the whole study period of 14 days and 24 hours, respectively. In Physioneal and Nutrineal aztreonam was stable at 6 °C for up to 14 days. Antimicrobial activity was retained in all PD fluids over the whole study period. Aztreonam remained stable and was compatible with the PD fluids, particularly with Extraneal or Nutrineal, and no compensatory dose adjustment is needed when stored for up to 14 days at refrigeration temperature before use. Bacterial peritonitis is a serious complication of peritoneal dialysis (PD) which often leads to membrane failure and patients discontinuing from peritoneal dialysis and switching to hemodialysis 1. The most common bacteria isolated from patients with PD associated peritonitis (PDAP) are primarily Gram-positive cocci like Staphylococci or Streptococci, but also Gram-negative bacteria like Escherichia coli or Pseudomonas aeruginosa are frequently detected 2. Particularly P. aeruginosa peritonitis is generally severe and associated with higher frequencies of hospitalization, catheter removal and transfer from PD to hemodialysis (HD) 3-5. Thus, empiric antimicrobial therapy should cover both, the most likely Gram-positive and Gram-negative bacteria. Aztreonam is the only clinically available monobactam with its antibacterial spectrum limited to Gram-negative aerobic bacteria and could be of strong use for the treatment of Gram-negative PDAP. In case of a P. aeruginosa peritonitis the current International Society for Peritoneal Dialysis (ISPD) guidelines suggest treating with two antibiotic agents with different mechanisms of action for three weeks 1. Therefore, aztreonam might be a feasible alternative to cephalosporins or fluoroquinolones, especially due increasing rate of multidrug-resistant Gram-negative bacteria 6. Intraperitoneal (i.p.) administration of drugs is preferable to intravenous or oral routes in PDAP due to the markedly higher concentrations achieved at the target site 1. In the past, aztreonam was assumed to be more cost intensive and more difficult to handle in continuous ambulatory peritoneal dialysis (CAPD), especially compared to fluroquinolones, since it must be administered four times daily 7,8. However, more recent studies have shown that once a day i.p. administration provides sufficient serum...