http://www.refillinstructions.com/Lexmark/L7.htm
Lexmark offers for sale two different versions of the same cartridge:
The regular version (#23, #24, #28, #29, #41, #42) is not refillable. It is the "Return Program" or "Prebate" cartridge. Supposedly, when you buy this cartridge you agree to return the empty cartridge to Lexmark. In return, Lexmark gives you roughly $5 off the retail list price of the cartridge.
The "A" version (#23A, #24A, #28A, #29A, #41A, #42A) can easily be refilled. However, the price of the cartridge is roughly $5 higher than the regular version, so most people unknowingly buy the regular version.
If you want to refill, you will need "A" version cartridges.