Human matrix metalloproteinase-26 (MMP-26/endometase/matrilysin-2) is a newly identified MMP and its structure has not been reported. The enzyme active site S1′ pocket in MMPs is a well defined substrate P 1′ amino acid residue-binding site with variable depth. To explore MMP-26 active site structure-activity, a series of new potent mercaptosulfide MMP inhibitors (MMPIs) with Leu or homophenylalanine (Homophe) side chains at the P1′ site were selected. The Homephe side chain is designed to probe deep S1′ pocket MMPs. These inhibitors were tested against MMP-26 and several MMPs with known x-ray crystal structures to distinguish shallow, intermediate, and deep S1′ pocket characteristics. MMP-26 has an inhibition profile most similar to those of MMPs with intermediate S1′ pockets. Investigations with hydroxamate MMPIs, including those designed for deep pocket MMPs, also indicated the presence of an intermediate pocket. Protein sequence analysis and homology modeling further verified that MMP-26 has an intermediate S 1′ pocket formed by Leu-204, His-208, and Tyr-230. Moreover, residue 233 may influence the depth of an MMP S1′ pocket. The residue at the equivalent position of MMP-26 residue 233 is hydrophilic in intermediate-pocket MMPs (e.g. MMP-2, -8, and -9) and hydrophobic in deep-pocket MMPs (e.g. MMP-3, -12, and -14). MMP-26 contains a His-233 that renders the S1′ pocket to an intermediate size. This study suggests that MMPIs, protein sequence analyses, and molecular modeling are useful tools to understand structure-activity relationships and provides new insight for rational inhibitor design that may distinguish MMPs with deep versus intermediate S1′ pockets.