Peritoneal dialysis (PD) fluids are known to suppress the reactions of inflammatory cells in vitro. PD-fluids have also been shown to have cytotoxic influence on mesothelial cells. The combinations of these factors may have a detrimental effect on the peritoneum or may impair cellular defence against bacterial peritonitis. Some authors have discussed the relevance of heat sterilization to both so-called peritoneal side-effects and to chemical decomposition of fluids. Four commercial PD-fluids and one laboratory-made PD-fluid were tested for cytotoxicity on a cultured fibroblast cell line, L-929. Cytotoxicity was determined as an inhibition of cell growth by quantification of total protein. The laboratory-made PD-fluid was sterilized either by filtration or by filtration and heat. The commercial and the heat-sterilized laboratory made PD-fluids caused significant inhibition of cell growth (53 to 76%) in contrast to saline and the filter-sterilized laboratory-made PD-fluid. Since the pH values of all the testsolutions were neutral, low pH was not the cause of toxicity. Our results regarding the L-929 cells indicate that the cytotoxicity of PD-fluids is of a general nature. Furthermore, the results indicate that the heat sterilization process might be partially responsible for causing toxicity in PD-fluids.