Non-competitive, sandwich enzyme immunoassay for both penicillin amidases from Escherichia coli is described. The assay involves the use of monospecific antibodies and their conjugates. The amidases inactivated by heating and by acid- or alkali-treatment cannot be assayed. Reproducible results for each amidase were achieved within 5-6 hours in the range of 3-500 ng/ml (0.25-40 mU/ml) and coefficient of variation was 13%.