Abstract: A sealing cap and container combination primarily intended for use in medical applications. The sealing cap of the preferred embodiment includes a central dome-shaped portion, annular portion, and sealing portion. In use, these portions are initially received in the mouth of the container and assume an initial concave shape with the sealing portion of the cap abutting and sealing against the inner surface of the container mouth. Thereafter, as the pressure within the container increases, the concave dome first moves toward a flattened shape and then inverts to a convex shape. As the pressure further increases, the annular portion of the cap then moves to a flattened position substantially perpendicular to the axis of the cap. During all this controlled movement, the sealing force of the cap progressively increases from an initial force to a maximum sealing force.