The Actinomadura R39 DD-peptidase is a very penicillin-sensitive enzyme. After a short time of contact with a liquid sample containing a beta-lactam antibiotic, the determination of the residual enzyme activity allows estimation of the concentration of the antibiotic in the sample. An alternative method utilizes the C-terminal domain of the Bacillus licheniformis penicillin sensor BlaR that is devoid of enzymatic activity. In this case, the protein is labelled with gold particles and the unreacted portion is captured by beta-lactam molecules covalently attached to a solid support. Both methods have been quite successful for the rapid estimation of beta-lactam antibiotics in milk.