you can use trypan blue, but it's quite subjective results. or you also use wst cell proliferation. http://www.sigmaaldrich.com/catalog/product/roche/cellproro?lang=en®ion=GB principle same as mtt assay.
In vitro spectrophotometric MTT assay - MTT (Sigma-Aldrich) was dissolved in RPMI 1640 medium, at a concentration of 5 mg/ml, and filtered through a 0.2 µm filter. Subsequently, 100 µl of the yellow MTT solution was added to each well of a 24-well plate, containing 1 ml of the cell suspension. The cells were then incubated at 37°C with 5% CO2. A blank solution was prepared according to the above procedure using complete medium without cells. After a three-hour incubation period, the resulting formazan crystals were dissolved with 1 ml of acidified isopropanol (0.05 N HCl in absolute isopropanol), and the absorbance of the obtained solution was measured at a wavelength of 570 nm using a Pharmacia Ultrospec III spectrophotometer (Pharmacia LKB Biotechnology).
IC50 value determination - Based on the obtained data using the in vitro MTT assay, the half maximal inhibitory concentration IC50 values for the TESTED AGENTS, were calculated separately, at 48 h after the human leukemia cells exposure to the action of these compounds. The IC50 values, were calculated
All te answer are good help for you but the MTT assay is just a redox-status assay of the cells and even if related to cellular viability, it is not a viability assay (in term of cell membrane permeability