Abstract: An improved assay for measuring the quantity of Beta-lactam antibiotics, particularly valuable for measuring penicillin in milk. The assay includes the covalent-attachment of a penicillin to a penicillin-binding-protein bound to a matrix, the highly selective attachment of an antibody to such a conjugate, and the detection and measurement of the quantity of resulting antibody-complex material as a direct measure of the amount of penicillin in the milk being tested.