In a collaborative study by seventeen laboratories, six aqueous sample solutions also containing 0.9% (w/v) NaCl were analysed for lactate and creatinine using isotachophoresis. A split-level study was carried out with three levels of the order of 3, 10 and 30 mmol/l, two sublevels and without blind duplicates. A calibration graph was constructed at five concentrations, using citrate and Tris as internal standards, added on a mass basis. The concentrations in the sample were determined in singular. After elimination of a few outliers using the Grubbs test at a 1% confidence level, data were treated according to ISO 5725. For low, medium and high concentration levels, the repeatability values r were 0.41, 0.40 and 1.67 mmol/l, respectively, for lactate and 0.63, 0.53 and 1.43 mmol/l, respectively, for creatinine. The reproducibility values R were 0.59, 1.12 and 2.05 mmol/l, respectively, for lactate and 1.33, 0.88 and 2.66 mmol/l, respectively, for creatinine.