Free aldehyde groups in tissue. Float sections on 0.5M NH4Cl for 1 hour before incubation. Antibody concentration too high. Gold conjugate concentration too high. Tissue poorly fixed.
Damaged cells stain nonspecifically.
Look at this first. I have no explanation for this and offer my apologies for complicating the field further. There is a section on our CCI home page concerning problems with antibody labeling if anyone is interested. Thank you to all those who re-posted the information on muliple antibody labeling protocols using monoclonal antibodies.