I've been working with some proliferation assays for a while, using 0.1% DMSO for my positive control. However, on several assays, it doesn't work (i.e., I get higher cell numbers than my low control, but not nearly as high as they should be). Recently, I had four assays tested in one day - all treated at the same time, same cells, same incubation, same evaluation, but the high control worked on only two of the assays. I use the same DMSO as control and to dilute my compounds - the compounds all behave as they should, so it shouldn't be the stock solution. No one knows what is causing the problem - I've cleaned the pipettes, used tips that prevent contamination by aerosolized compounds, checked my protocol. Is there any reason why this is happening? Any help would be greatly appreciated, thanks!!