The radiochemical purity of preparations for 99mTc-bone-scanning was tested using thin-layer chromatography and electrophoresis. The investigation included ten commercial kits, comprising four types of phosphorus compounds, i.e. sodium polyphosphate, sodium pyrophosphate, disodium diphosphonate, and sodium monofluorophosphate. Out-spoken differences in the content of free pertechnetate were found between the different kits. The contents in preparations from the same kit were fairly constant, but in some cases a change could be referred to the age of the 99mTc-generator or the time of analysis. No significant difference was found between preparations made from eluate with and without oxidizer. A 99mTc-tin-colloid was sporadically present in the preparations.