The evolution of extracorporeal treatment of end-stage renal failure has enforced focus on the purity of dialysis fluid. A major challenge of high-flux haemodialysis (HD) and haemodiafiltration relates to the necessity for ultrapure dialysis fluid and for sterile non-pyrogenic substitution fluid. The present review focuses especially on the possible microbial contamination including, next to intact micro-organisms, a variety of microbial derivatives. It is pointed out that there are conditions (e.g. contamination by non-culturable micro-organisms or bacterial derivatives other than lipopolysaccharides) where the detection of biologically relevant contaminants can be missed when applying the recommended standard detection methods such as bacterial culture and limulus amoebocyte lysate test. Possible approaches for action upon positive sampling results, exceeding the levels recommended in the latest ISO 11663:2009, are described in detail and illustrated with flow charts. The issue of purity of dialysis fluids is highly relevant, since the chronic exposure of HD patients to low levels of cytokine-inducing microbial components can significantly contribute to the micro-inflammatory status of these patients.