For reasons best known to themselves you cannot fit Lexmark cartridges into a Dell Printer. If you press programmes then select Dell 720, one of the options is order Dell cartridges, or they are available from Dell online. They are not the cheapest items to purchase so I got bottles of refill ink and have had no problems. You can get the instructions for opening and filling the cartridges by e-mailing
www.jr-inkjet.com. If you live in the States,
www.atlanticinkjet.com have some of the best prices I've seen for refill ink, I had some sent to the UK and after courier delivery and import taxes, it was still 50% cheaper than getting it here.