In a preferred embodiment, the ink reservoir in a print cartridge consists of a spring-loaded collapsible ink bag, where the spring urges the sides of the ink bag apart and thus maintains a negative pressure within the ink bag relative to ambient pressure. An ink refill system containing a supply of ink has a valve with a connector portion which is engageable with the connector portion of the print cartridge refill valve. When the valves are connected, the negative pressure within the print cartridge ink bag draws the ink from the ink refill system reservoir into the ink bag until the ink bag is substantially full. The print cartridge is then removed from the ink refill system. The mechanical coupling initially created between the two valves acts to pull the two valves closed as the print cartridge is pulled from the ink refill system. Once the two valves are closed, further pulling of the print cartridge releases the mechanical coupling, and the print cartridge may now be reused. Various external ink supply structures, having a variety of types of ink exit ports, are disclosed for use with the preferred print cartridge. In a preferred embodiment, the ink refill system contains one recharge for the print cartridge.