1. Inbred mouse strains differ markedly in the expression of a kidney brush border metalloendopeptidase, meprin-a. 2. Brush border preparations from mice of the low-meprin-a phenotype (specific activities less than 5% of the high-meprin-a trait) contain a metallo-endopeptidase, meprin-b, that is larger than meprin-a, and which is inactive unless the membrane preparations are treated with trypsin. 3. This cryptic metallo-endopeptidase has been previously postulated to be a stalled precursor of meprin-a. 4. We show here that meprin-b is present in all mice-high and low meprin-a phenotypes--and that this activity is similar in substrate specificity and amount present in the brush border. 5. Meprin-b may therefore be a distinct gene product that is independent of meprin-a phenotype.