Epidermin is a 21-amino acid lantibiotic that has antimicrobial activity toward gram-positive bacteria. It is produced and processed through a series of proteins to form the mature epidermin. EpiP is one of the proteins in the cascade to produce this mature lantibiotic. EpiP is intracellularly produced and is then transported across the cell membrane where it is attached to the surface of the cell. It is puzzling as to why Staphylococcus aureus has this protein since it does not produce epidermin and is lacking several of the genes for its production. The EpiP protein from Staphylococcus aureus shows similarity to subtilisin-like serine proteases, and has conserved residues (Asp140, His 187, and Ser393) corresponding to the catalytic triad for serine proteases. In our wild-type structure the polypeptide chain has been cleaved somewhere between residues 95 and 100, indicating that the cleavage may have been autocatalytic. To determine if the protein is acting like a subtilisin-like serine protease, we mutated the hypothesized catalytic serine residue to alanine. The crystal structure of this mutant protein shows that the polypeptide chain is not cleaved and is not interacting with the active site.