Since I have purified my protein of the interest using denaturing method, I had to get rid of urea before mice injection. I tried to dialyze my protein against reducing concentrations of urea in PBS. The dialysis has been done successfully as no aggregation happened. But when checking the protein concentration using Bradford assay, not only color change was not happened by adding the protein sample to Bradford reagent but also the concentration reported by the spectrophotometer was zero! However, after running the dialyzed protein on the SDS-PAGE the band of protein was visible! There were no detergent or other Bradford interferer in the protein elution buffer or in PBS. Could anyone help me with this problem?