The effectiveness and tolerance of the new semisynthetic acylureidopenicillin 6-[(R)-2-[3-methylsulfonyl-2-oxo-imidazolidine-1-carboxamido]-2-phenyl-acetamido]-penicillanic acid sodium salt (mezlocillin, Baypen) were investigated in 10 patients with bone and soft tissue infections of the mouth and jaw. The antibiotic was given in doses of 2 to 10 g/d. Indications for treatment were osteomyelitis of the jaw and/or phlegmon of the floor of the mouth and abscesses of the soft palate. 9 out of 10 infections could be cured. Due to the short treatment period one case was not assessable. In 6 patients 9 causative aerobic and anaerobic organisms were isolated which were eliminated. The clinical and laboratory control examinations did not reveal any adverse reactions.