Extracellular lytic endopeptidases AlpA and AlpB of the Gram-negative bacterium Lysobacter sp. XL1 have a high degree of homology and are synthesized as preproproteins consisting of a signal peptide, a propeptide, and the mature protein. In the present work, two monoclonal antibodies against the AlpA propeptide (ProA) and eleven antibodies against the AlpB propeptide (ProB) have been obtained. The affinity constants for antibodies to ProA were 2.9 × 109and 3.5 × 109M-1, and those for antibodies to ProB were from 1.5 × 108to 2.2 × 109M-1. The antibodies showed no immune cross-reactivity with each other and with mature forms of the enzymes. On the basis of monoclonal antibodies, a sandwich enzyme immunoassay has been developed, which makes it possible to detect these propeptides in the dissolved native form. The linear range of the detection of ProA was 1.5-100.0 ng/mL with an error of measurement of 6%, and that of the determination of ProA was 0.2-6.25 ng/mL with an error of measurement of 6%. By using the assay, propeptides ProA and ProB were detected in cell lysates of Lysobacter sp. XL1 in an amount of 1.18 ± 0.03 and 0.096 ± 0.002 ng per 1 OD540 of bacterial culture, respectively. The immunochemical assay for the detection of different forms of AlpA and AlpB can be useful in solving the problems associated with their secretion into environment. © 2014 Pleiades Publishing, Ltd.