The formation and stability of the complex between bumetanide (3-butylamino-4-phenoxy-5-sulphamoylbenzoic acid) and Fe(III) ion was studied. It has been found that bumetanide reacts with Fe(III) chloride in the presence of ammonium thiocyanate at the pH 1.83-1.92 to form a violet complex soluble in chlorophorm, with the maximum absorbance at 514.5 nm. At the optimum experimental conditions the conditional stability constant of the complex was found to be log K' = 7.34. The Beer's law was obeyed up to the 160 microg x cm(-3) and the relative standard deviation varied from 0.406% to 0.475% (n = 10). The proposed method was found to be suitable for the accurate and sensitive analysis of bumetanide as a pure compound and from tables and ampoules.