On the basis of reports of an increased incidence of postoperative sensitivity associated with the use of a particular spherical amalgam alloy, an investigation of in vitro marginal leakage was initiated. An air pressure test was applied to simulated Class I restorations made from this alloy and a control alloy. Factors such as mix plasticity, condensation force, and condensation technique also were examined. The results showed that the spherical alloy in question had a greater tendency for marginal leakage than the control alloy. For both alloys studied, the following factors are important in reducing marginal leakage: increased plasticity of the mix; increased condensation force; condensation of each increment directly along or against cavity walls in overlapping thrusts; and burnishing the amalgam after condensation. Although it is not known what percentage of postoperative sensitivity is caused by leakage of amalgam restorations, the results of this study indicate that leakage can be minimized by using optimum amalgam manipulative procedures and that a reduced incidence of postoperative sensitivity is likely to result.