System and method for mixing the contents of two containers

A port assembly for establishing fluid communication between a first container containing a first substance and a second container containing a second substance, the port assembly comprising:

• a retainer for connecting a first container;
• a port housing defining a cavity, the retainer positioned within the cavity, the port housing comprising an axially fixed actuator constructed to force a stopper associated with a first container into a first container;
• and
• a plug member constructed to seal a fluid passageway between the port housing and an interior of a second container, the plug member configured to move axially relative to the actuator,
• wherein the retainer is configured to rotate relative to the port housing, wherein rotation of the retainer relative to the port housing causes the retainer to move axially relative to the port housing, whereby the actuator forces a stopper associated with a first container connected to the retainer into a first container, and whereby rotation of the retainer relative to the port housing further causes the plug member to move to an open position in which it does not seal a fluid passageway between the port housing and an interior of a second container.