A new and sensitive spectrophotometric method has been developed for the determination of tetracyclines either in a pure form or in Pharmaceuticals, by a molybdenum blue method. The procedure is based on the observation that, in sulphuric acid medium, tetracyclines reduce ammonium molybdate to molybdenum blue, the absorbance of which is proportional to the amount of antibiotic present. The variables affecting development of the color have been investigated and the conditions optimized. Beer's law is obeyed for up to 20 μg/ml of tetracycline HCl and oxytetracycline HCl, 28 μg/ml of demeclocycline HCl, 18 μg/ml of chlortetracycline HCl, 32 μg/ml of doxycycline HCl and 40 μ/ml of rolitetracycline. Molar absorptivities (1 mol−1cm−1) and Sandell's sensitivities (μ cm−2per 0.001 absorbance unit) are, respectively: tetracycline HCl 4.9×104and 0.0098, oxytetracycline HCl 5.4×104and 0.0092, demeclocycline HCl 1.6×104and 0.0313, chlortetracycline HCl 5.5×104and 0.0094, doxycycline HCl 3.4×104and 0.0141, rolitetracycline 2.7×104and 0.0195.