What is the ideal time to add drugs when doing MTT assay - (Apr/05/2013 )

i would like to know when is the ideal time to add drug to the cells, when doing MTT assay,
i have read in papers like at the same time, some papers suggest 24hrs after culturing in 96well plate.
should i wait for the cells to be concurrent to add the drug?
what is the significance of initial cell number in MTT assay.

please kindly help me with a suggestion.
thank you very much

Depends on the drug and the cells more than anything -what sort of cells are you using? what does the drug do? How many cells are you plating? How would confluency affect the action of the drug?

I m using promegas cell titre aquoeus one solution reagent.
before when i used MTT, i used to get consistent results..
but with cell titre aquooeus one solution reagent, m not getting good result..
i use lung cancer cell lines, and treat them with different concentrations of cisplatin
after 24hrs treatment with the drug, i could see the cells died at higher concentrations of cisplatin, but after adding the reagent all wells shows same colour, and there is not much difference in the OD values also..

please help with a suggetion

Check that the drug solvent (DMSO?) is compatible with the kit you are using.

with hello

i suggest 24hrs after culturing in 96well plate because you must give time to cells for sitting