Sensitive and selective, but fast and simple assays of cyanide ion in waste and natural waters, as well as in biological samples are required in case of accidental pollution with such species. Cyanide reaction with ninhydrin has been recommended for instant detection of small traces of cyanide (detection limit 1.5 ng ml -1 ). However, the oxidizers and heavy metal ions, such as Hg, Cu, Ag may interfere with the determinations. Therefore, we report here cyanide determination with ninhydrin and its usage in monitoring polluted water with heavy metals. The effect of heavy metals and SH-compounds were studied in order to increase the method range of use. N-acetyl cysteine proved to be a selective reagent for protection of cyanide detection and determination against the interference of heavy metal ions.