Abstract: A method for resupplying a cassette assembly (14) for an ambulatory medical infusion pump (10) with a new reservoir bag (22a) includes the steps of substantially emptying a used reservoir bag (22) located within said cassette interior (27) of any fluid, supplying an access slot (40) to the interior (27) of the cassette large enough to withdraw the used drug reservoir bag (22) from within the cassette, removing a luer lock (34) located at the end of microbore tubing (32) attached to the reservoir bag, removing the tubing (32) from under a guide retainer (30) in a pumping plate (16) of the cassette, withdrawing the reservoir bag (22) from the access slot (40), inserting a new unused reservoir bag (22a) into the access slot (40) to the interior of said cassette, by inserting a tube (32a) of a new unused reservoir bag and pulling the tube (32a) such that the bag (22a) is pulled through the slot (40) and into the interior (27) of the cassette shell (18).