An immunologically compromised patient was found to have a postoperative intra-abdominal abscess from which Staphylococcus epidermidis was the sole isolate. Studies of the isolate in a rabbit and in mice showed no evidence for unusual virulence of the organism. The S epidermidis probably was introduced into the peritoneal cavity during surgery and, because of the patient's neutropenia, produced a serious infection within a few weeks. To our knowledge, this is the first case of a intra-abdominal abscess associated with S epidermidis to be described in the literature.