Using an in vitro air pressure test, the authors found a wide variation in the microleakage characteristics of amalgam restorations made from 33 recently manufactured precapsulated alloys. Spherical particle alloys as a group exhibited the highest microleakage values. When the authors changed the mercury/alloy ratios to produce 1 percent more mercury in the capsules by weight, microleakage decreased significantly. Because extensive microleakage of amalgam restorations has been related to increased postoperative sensitivity, the authors suggest that practitioners should be alert to this potential problem.