Antibiotic audits that were conducted within our hospital between 1978 and 1982 showed persisting patterns of inappropriate antibiotic use. A commercial advertising agency was commissioned to plan a campaign to change entrenched prescribing habits. Amoxycillin was chosen as the test drug because previous audits consistently had shown that its intravenous administration was prescribed more frequently than was that of benzylpenicillin in the treatment of primary pneumonia. In addition, amoxycillin given by mouth was prescribed six-hourly rather than eight-hourly in one-third of the patients who were surveyed. A three-month remedial campaign used direct mail (pads and pens), display stands and posters, all of which focused on the booklet Antibiotic guidelines. Educational material was placed wherever staff members congregated. At the end of the campaign, intravenously administered amoxycillin was used in only 8% of 50 patients with primary pneumonia and amoxycillin given by mouth was prescribed six-hourly in only 10% of 99 patients. These changed habits were sustained six months after the campaign but showed some fall-off 18 months after the campaign. The campaign costs of $10,000 were recouped within 12 months by savings on drug costs. It is concluded that educational advertising is an effective means of changing prescribing behaviour in a hospital.