The present study was focused to develop an analytical method to analyse â-lactam antibiotics present in aqueous solutions and able to be applied directly in the fields. The method here presented is based on the chemical reaction between â-lactam antibiotics with phosphomolybdic acid, a heteropolyacid able to be reduced by the thiols obtained after the acid hydrolysis of the â-lactam antibiotics. The blue product obtained, which is proportional to the antibiotic amount, is adsorbed on polyurethane foam, and the intensity of color is directly evaluated on the solid material by means of diffuse reflectance spectroscopy. For all six evaluated â-lactam antibiotics (amoxicillin, ampicillin, penicillin V, penicillin G, cefotaxime, cefuroxime), the analytical parameters achieved were: LOD = 0.03 µg/ml, LOQ = 0.1 µg/ml, RSD = 4.0 %. The study of possible interferences showed that using solid phase extraction is possible to eliminate even strong interferences as ascorbic acid, cysteine and reducing salts like stannous chloride. The present method has avoided, as much as possible, the use of expensive and contaminating solvents and chemical reagents. The method presents the possibility to be applied using only a desktop scanner and a personal PC for measuring the color of the adsorbed material on the PUF.