If the volume is >500 ul you can simply use dyalisis tubing. Do not dialize directly against buffer. It is better if the urea concentration is reduced gradually (for example, against 6M, then 3M and finally buffer alone). Good luck!

If you can get them, D-tubes from Millipore are very practical, obviously more expensive than dialysis tubing but are ready to go and its as easy as loading an eppendorf! Definitely do the gradient reduction of Urea concentration Lucretia suggested and bear in mind that your protein might crash if you fully remove the Urea, try to check in each concentration to see. Depending on the application some amount of urea is tolerable.

Thank you guys! very good advices

I'm planning use this protein as antigen in a luminex platform. So far, playing with coupling conditions, I'm able to get an optimal coupling of my antigen (in urea 8M) into magnetic beads but I was wondering if it would be possible increase the coupling performance removing or decreasing the urea concentration of my protein elution.

I had some success with dialysis against my favorite buffer (carbonate buffer for coupling!) containing high salt (0,5M) and 0,1% or less SDS..to prevent aggregation.

I am surprise you successfully coupled your prot on mag beads in buffer containing 8M urea (amine prevent coupling with CNBR chemistry!....can you give us you protocol!!!