Two different UV-spectrophotometric modes, zero-order and first-derivative, have been applied for direct quantitation of aztreonam in bulk form and in some of its pharmaceutical formulations. Direct UV-measurement of aqueous solutions of the drug at about 285 nm exhibits significant linearity at a concentration range 1–5 mg/100 ml with a relative standard deviation ± 0.4%. The first-derivative (d'A) spectrophotometric measurements of the drug at 300 nm show results with relative standard deviation ± 0.6%. Drug assay gives percentage contents of 100.29 ± 0.95 and 99.16 ± 0.80 by applying zero-order spectrophotometry and 98.95 ± 0.52 and 98.70 ± 1.31 by adopting derivative spectrophotometry for aztreonam in 0.5-g and 1.0-g vials, respectively. The reproducibility and accuracy of both described methods have been assessed by applying the standard addition technique of the pure drug to each pharmaceutical formulation. The percentages of the mean recoveries of added drug were 100.28 ± 1.61 and 100.17 ± 0.90 for zero-order measurements and 100.02 ± 1.82 and 99.65 ± 1.05 for first-derivative spectrophotometry in case of 0.5-g and 1.0-g injectable samples, respectively.