Commercial heat-sterilized fluids for peritoneal dialysis (PD) are unphysiological due to low pH, high osmolality, and the presence of several toxic glucose degradation products, formed during heat sterilization. These properties are all believed to negatively affect host defense in patients. Both the low pH and the high osmolality are known to equilibrate after being introduced into the abdominal cavity of patients. Cytotoxicity due to glucose degradation products has, however, not yet been studied in this respect. Effluent peritoneal dialysates were collected from patients after dwell times of 0, 5, 15, 30, 120, and 240 minutes. Cytotoxicity, measured as the inhibition of cell growth of a cultured fibroblast cell line (L929), osmolality, and pH were determined. PD fluids, due to the presence of degradation products, were found to remain cytotoxic after a dwell time of more than 30 minutes. However, after 4 hours no cytotoxicity could be observed in the effluent fluids. Osmolality slowly decreased during the entire dialysis period, while pH rapidly increased and was close to neutral within 5 minutes following instillation. It is concluded that the presence of glucose degradation in PD fluids may be as important as low pH and high osmolality for clinical complications.