An ink priming system for a cartridge unit of the type having a pagewidth printhead assembly in fluid communication with one or more ink storage compartments. The system includes a priming inlet provided on said pagewidth printhead assembly for receiving a supply of ink for priming the cartridge unit; an ink flow passage providing fluid connection between the pagewidth printhead assembly and one of the ink storage compartments; and an ink retaining member located within each ink storage compartment for receiving and storing ink. Ink is supplied to the priming inlet of the pagewidth printhead assembly to flow from the pagewidth printhead assembly to the ink storage compartment via the ink flow passage to prime the ink retaining member of each ink storage compartment with ink.