Customer Rating: 
Summary: Does NOT work with MFC-420CN
Comment: While I can't promise that this cartridge won't work with any MFC-420CN (one of the previous reviewers claims it will), I can say that this cartridge did not work on my MFC-420CN. As soon as I swapped it out for a LC41BK, it worked fine. Since the LC41HYBK packing does NOT mention the MFC-420CN, I would avoid this ink if that's the printer you have.
Customer Rating:
Summary: Works fine with MFC 420CN
Comment: No problems with the LC41HYBK in the MFC 420CN, even though MFC420CN is not listed on the packaging of the LC41HYBK.
April 6, 2006: Apologies to J. Miller regarding LC41HYBK compatability with MFC 420CN. However, I have double checked the markings on the current black cartridge in my MFC 420CN printer , it is a LC41HYBK , and it is working fine. Obviously all MFC 420CN's are not the same.
Customer Rating:
Summary: Beware
Comment: The description is wrong for other printers/mfc as well. The Brother website lists ONLY the following printers as compatible:
FAX1840C, FAX1940CN, FAX2440C, MFC3240C, MFC3340CN, MFC5440CN, MFC5840CN
Customer Rating:
Summary: Does not support MFC210C!!!
Comment: Be careful people! It doesn't support MFC210C although the product description at Amazon says so. Actually the box for the toner doesn't list MFC210C. So this is a mistake at Amazon website. I have to return it and buy model LC41BK.
Summary: Does NOT work with MFC-420CN
Comment: While I can't promise that this cartridge won't work with any MFC-420CN (one of the previous reviewers claims it will), I can say that this cartridge did not work on my MFC-420CN. As soon as I swapped it out for a LC41BK, it worked fine. Since the LC41HYBK packing does NOT mention the MFC-420CN, I would avoid this ink if that's the printer you have.
Customer Rating:
Summary: Works fine with MFC 420CN
Comment: No problems with the LC41HYBK in the MFC 420CN, even though MFC420CN is not listed on the packaging of the LC41HYBK.
April 6, 2006: Apologies to J. Miller regarding LC41HYBK compatability with MFC 420CN. However, I have double checked the markings on the current black cartridge in my MFC 420CN printer , it is a LC41HYBK , and it is working fine. Obviously all MFC 420CN's are not the same.
Customer Rating:
Summary: Beware
Comment: The description is wrong for other printers/mfc as well. The Brother website lists ONLY the following printers as compatible:
FAX1840C, FAX1940CN, FAX2440C, MFC3240C, MFC3340CN, MFC5440CN, MFC5840CN
Customer Rating:
Summary: Does not support MFC210C!!!
Comment: Be careful people! It doesn't support MFC210C although the product description at Amazon says so. Actually the box for the toner doesn't list MFC210C. So this is a mistake at Amazon website. I have to return it and buy model LC41BK.