Abstract: A rapid single step assay suitable for the detection or quantification of .beta.-lactam antibiotics and .beta.-lactamases. The assay can be performed directly on samples of food, such as milk and meat, blood or serum and is useful in determining the suitability of a particular antibiotic in treating a particular bacterial infection and in diagnosis of a bacterial infection. The assay is also useful in determining and quantifying .beta.-lactam antibiotic resistance. The assay can be performed on an IGEN Origen.sup.R Analyzer.