A seal for use in a surgical instrument to provide a gas-tight seal with an instrument passed through the seal. The seal can form a gas-tight seal with an instrument having a diameter within a wide range of diameters. The seal comprises a seal body, an instrument seal, and a laterally-compliant seal mounting device. The seal body includes a bore through which the instrument is passed. The instrument seal is made of an elastic material and is disposed substantially perpendicular to the axis defined by the bore in the seal body. The instrument seal includes an instrument port through which the instrument is passed. The laterally-compliant seal mounting device mounts the instrument seal to the seal body, forms a gas-tight seal between the instrument seal and the seal body, and allows the instrument seal to move freely laterally in response to lateral movement of the instrument.