problems with MTT assay - (Jul/07/2009 )

Hi, I'm making assays with mtt on LNCaP and DU-145 cells, in order to evaluate de cytotoxity and proliferetion effects of a plant extract. The problem is when I evalute the extract on LNCaP cells, the absorbance (read with a D.O. 570 nm) of the highest dosis of my extract (1 mg/ml) on 24 hrs of treatment, the absorbance of this cells is 3 times higher than the control (cells only with medium), but when I look this in the microscopy I see cell "deads" with the treatment. I thought the possibility that my extract can interfere with the MTT test, nevertheless this problem doesn't occur with the DU-145 cells at the same dosis and time. Thank you for the help.

Do the same with extract and cells. Instead of adding MTT , Do trypan blue exclusion test .