An automated analytical system has been used to determine the amount of oxytetracycline in pharmaceutical products and fermentation broth at a level of 0.1 mg/mL. The method is based on the formation of a colour complex between the oxytetracycline and ammonium molybdate in acid media at 80 degrees C. The effect of temperature, concentration of HCl and ammonium molybdate upon the stability of the complex was determined. Standard error of difference is 0.43% for pharmaceutical products and the fermentation broth.