Mice Serum was harvest and run SDS-PAGE gel, membrane was incubated by anti-tubulin antibody (sigma), Why are there tubulin bands? Is tubulin secreted into serum?
I agree with the above, lysed cells, damaged tissue etc will all contribute to the problem you are seeing. I wonder if you use a cannula to obtain the serum you might avoid the issue of local tissue damage at the site of hypodermic puncture and therefore reduce contamination?