Hi Lahoucine , what is the amino acid similarity between the two proteins? Unless it is nearly identical (and as it is secreted) I would personally look into getting or making a species specific Elisa.

I would use either Flag, myc or HA epitope tags.  They are all small enough so that generating an immune response should not be a major problem.  You would likely have to put them at the C-terminus because of the signal peptide at the N-terminus.  More importantly, how will you express the gene?  Can you make transgenic hamsters?

I would not worry about any immunological reactions, but it is more important to wonder if the function of your specific protein can be affected by the addition of a small epitope tag. You can either fuse the tag to the C-terminus, or inbetween the signal peptide and the mature region (N-terminal), or even internally when you know of a cytosolic loop in your protein structure. If the C-terminus is exposed on the surface, that's where I would put the tag. If not, check if the N-terminus is exposed. Addition of the tag to a surface exposed region is not needed for detection, because you denature your protein anyway when you do western blotting. However, if you add the tag to a region of the protein that is buried inside the folded structure, the tag can disrupt this structure and cause the protein to malfunction.

If you have no idea about the structure of your protein, then try both N-terminal and C-terminal tagging ( and remember the signal peptide is cleaved, so it goes at the N-terminus of the mature peptide, and C-terminal of the signal peptide).

First of I assume you want to do more than expressing tagged proteins those float around. I worked with Flag-, HA, and GFP-tagged proteins for various purposes. Working with small tags is better since you do not have to worry about any conformational changes those prevent protein-protein interactions, if this is what you intend to look at later. Tagging on 25kD is not unproblematic, though looks nice under the microscope. You should also use both, C-termanial AND N-terminal tagging, depending on the next experiments.