How to evaluate Cell viability - (Nov/08/2006 )

Hello Friends,
I am performing the MTT assay to determine the cell viability. I have done the experiment in 96 well plate format, with different drug concentration treatment with positive control and without treatment control.
I got the different OD s from spectrophotometry. Could you please tell me , that how could I get I get the viablity percentage from this assay?

Thanks
Deb