A spectrophotometric determination is described for cephalosporins, offering adequate sensitivity and good precision. The procedure applies successfully to a wide variety of cephalosporins, also in pharmaceutical preparations: cephalothin, cefacetrile, cephapirin, cefotaxime, ceftizoxime, cephaloridine, cefazolin, cefamandole nafate, cephalexin, cefadroxil, cefoxitin and cefuroxime. The method employs a reaction with ammonium molybdate in sulphuric acid medium. The antibiotic is heated at 91.5°C for 15 min and the absorbance of the coloured product is measured at 670 nm against a reagent blank treated similarly. Beer's law is obeyed up to 125 to 150 μg of cephalosporin in the 5-ml final solution. The effect of reagent concentration and reaction conditions are discussed.