Two metalloendopeptidases, meprin and endopeptidase-24.11 ("24.11"), were isolated from mouse kidney membranes, and their structural and catalytic properties were investigated. The enzymes both cross-react with antibodies prepared in rabbits against purified preparations of meprin; thus they share some immunologic determinants. Meprin and 24.11 have similar subunit molecular weights of 85 000 and 90 000, respectively, as demonstrated after sodium dodecyl sulfate polyacrylamide gel electrophoresis in the presence of 2-mercaptoethanol. However, under non-reducing conditions, meprin migrates as an oligomer while 24.11 remains monomeric. This and other data indicate that meprin subunits are linked by disulfide bridges, whereas endopeptidase-24.11 subunits are not covalently linked. Both endopeptidases hydrolyze insulin B chain and are totally inhibited by EDTA and o-phenanthroline. The activity of 24.11 against insulin B chain was totally inhibited by low concentrations of phosphoramidon (less than 2 nM), whereas meprin was not inhibited by concentrations of this inhibitor as high as 20 microM. Large proteins are not substrates for endopeptidase-24.11, while meprin degrades proteins such as azocasein rapidly (apparent Km = 0.65 mg/ml). Meprin appears to require an extended polypeptide chain in substrates while 24.11 prefers smaller peptides as substrates. Both endopeptidases have a preference for peptide bonds that contain hydrophobic amino acids. With the octapeptide angiotensin II as substrate, both enzymes hydrolyze the central Tyr-Ile bond; 24.11 also cleaves at Arg-Val and Ile-His. The two endopeptidases show many similarities immunologically, structurally and catalytically, however, they display distinct characteristics which may be physiologically important.