proliferation assay -96 well plate - null results?? (Sep/27/2010 )

I plated cells in 96 well plate and treated with various growth factors. Oddly enough my negative control, no serum or phenol-red media, had the highest reading compared to the other wells. I did this in triplicate on several plates and got the same results. I am using Biovision quick cell proliferation assay kit. Any thoughts out there about what could be the problem?

how long do you leave them for? I usually plate them one day then 72 hours later add MTT reagent. works fine.