Normal serums are lipid extracted to improve clarity, dialyzed against phosphate buffered saline (PBS) containing sodium azide, and freeze dried. Normal serum diluted to 5% (v/v) in PBS is strongly recommended as a blocking reagent to reduce background from non-specific, conserved sequence and/or Fc receptor binding. Best results are obtained with diluted normal serum from the same host as the labeled antibody, as a separate incubation step before addition of the primary antibody.