I have been treating a cell line with increasin concentrations of MG132 10-100uM).i make a 1mM stock solution in DMSO then dilute it down in media. however i am getting very variabe results each time. some times a falt response of all concentrations giving 40% cell death and other times ups and downs in the curvi dont understand why. i tried many times. i am using an LDH assay and an ATP assay

What cells are you using?
Do you use them at one consistent level (e.g. 70% confluency, 5% serum, 2-5 passage etc). Since MG132 is a very good proteasomal inhibitor - and as such is blocking a critical cell pathway (most proteasome component knockouts, and all functional ones, are lethal), you need to ensure that all other conditions are uniform.

Also, try titering across a wider range - some cell types are more sensitive than others. If there is no published literature for your cell type, I'd recommend a pilot assay with a 1000x range so that a sigmoid profile is seen.

thank you for your answers

yes i am using consistent cell confluence and other parameters. the problem is an increase in the concentration of MG132 within the same experiment is not giving me an increase of cell death. i have tried very low concentrations and very hig. in fact ive tried from 0.1 uM to 100 uM

I have used MG132 at 10uM at a dilution of 0.1%DMSO and I got good results with nearly no cell death.
The treatment was done on BHK and HeLa cells for 3Hr.
What cells are u using?
Guy