b-lactam antibiotics are among the most widely used antibiotics for the treatment of bacterial infections in man and animals. b-lactams are also administered to animals in feed for growth promotion and for collective prophylactic treatment. The monitoring of b-lactam residues in edible tissues and milk is important because of the hypersensitivity of some individuals to these antibiotics and also the emergence of antibiotic-resistant strains of bacteria. For this reason, maximum residue limits have been specified for food products and milk to try and control the levels of these antibiotics reaching the consumer.