A controlled cross-over study in 12 Victorian public hospitals was performed to examine the power of marketing techniques in influencing prescribing. The targeted prescribing behaviour was the use of antibiotic prophylaxis in surgery, and the criteria for judging the appropriateness of therapy were its duration and timing, as are detailed in the fourth edition of the booklet Antibiotic guidelines. The first intervention was mounted in 1985 in six hospitals (two metropolitan teaching hospitals, one suburban general hospital and three rural hospitals), and six matched hospitals acted as control hospitals. One year later, the intervention was mounted in the six hospitals that previously had been the control hospitals. The interventional campaign consisted of material that was similar to that which is used by the pharmaceutical industry, including an "academic" representative. Its effect was assessed by audits that were performed before and after the first interventional campaign and again, one year later, after the second interventional campaign. The proportion of antibiotic courses that were assessed as satisfactory in terms of duration increased significantly after the first campaign in the hospitals where the intervention was mounted. No significant changes in prescribing occurred in the control hospitals. In the hospitals which were control hospitals in 1985, and in which the intervention occurred in 1986, the proportion of antibiotic courses that were assessed as satisfactory also increased significantly after the interventional campaign. A fall-off in performance occurred during the 12 months after the campaign in the 1985-interventional hospitals. Calculated cost savings more than outweighed the costs of the campaign. We conclude that inappropriate prescribing behaviour in hospitals can be modified successfully by educational marketing techniques.