Biapenem is a broad-spectrum carbapenem used for the treatment of intra-abdominal infections and for antibacterial prophylaxis in abdominal surgery. (1) Since biapenem acts at the infection site, the drug's penetration into an intra-abdominal site such as peritoneal fluid (PF) is a key determinant of its efficacy. However, most pharmacokinetic studies on biapenem have focused on plasma concentrations. (2) The treatment of patients with an intra-abdominal infection would benefit from a more thorough understanding of the rate and extent of penetration into the abdominal cavity.