Hi, from my little experience working with MTT, I have observed that sometimes over-confluence of the well plate could account for  your observation. Also ensure that you are seeding the same number of cells per well in the treatment group as you have in the control. Then some  literature search on your extracts and the particular cell line you are using could also give you a clue. All the best.

Thanks Saheed!!!

Hi Ignacio!

I totally agree with Saheed! I would also add that depends on the treatment that you are using, the absorbance can be higher as the control: indeed, some compounds that you might test are absorbing as well (I had problems with cells incubated with nanoparticles that also absorbed at the same longwave than the one that you use for the MTT). I suggest you to add control wells without cells, but with the same media than the cells that you are measuring the viability.

In the worst case, I suggest you to use other viability tests, such as Alamar blue (well, just check it and find the one that fits better to your study!).

All the best