Immunofixation is a diagnostic and research tool used for characterizing the electrophoretic location of immunoglobulin fractions in serum and urine. Commercially available polyclonal antisera which discriminate two IgG subclasses (IgG1 and IgG2) are available and commonly used. More recently, four IgG subclasses have been defined in the dog based on cDNA data. Archived serum from 16 dogs with naturally occurring monoclonal or biclonal gammopathies were characterized using routine serum protein electrophoresis, routine immunofixation and LCMS/MS as 3 IgA, 3 IgM, 2 IgG2, 7 IgG3 and 2 IgG4 heavy chain predominant cases. Immunofixation reactivity of a panel of commercially available antisera to these cases was characterized. The anti-human IgG antisera was the only tested antisera which bound all canine IgG restricted bands without also labelling IgA or IgM heavy chains or light chains. The tested polyclonal antisera labeled as reacting with canine IgG2 bound canine IgG2, IgG3, IgA and IgM and may label IgG1. The tested polyclonal antisera labeled as reacting with canine IgG1 bound the canine IgG4 bands but not those identified as IgA, IgM, IgG2 or IgG3 and likely did not bind IgG1. This data suggests that commercially available polyclonal IgG1 antisera (Bethyl A40 - 120A and Bio-Rad AHP947) can be used to positively but possibly not selectively identify canine IgG4 by immunofixation.