GFAP-molecular mass

[Mano]

Hi all,
    I am using a polyclonal antibody to GFAP to determine if GFAP is
expressed in P19 cells during differentiation.  However, I detect a
major polypeptide of approximately 58 kDa using western blot.  P19 are
of mouse origin.  I used a positive control from rat primary astrocytes
which gives a major size of 50 kDa.  I am wondering if the 58 kDa
protein that I detect is GFP or a cross reaction with another protein...

please help.

Thanks.
Emmanuel