Nancy,

We've had good luck using a mouse anti rat CD31 from Serotec, MCA1334G.
However, our best results were obtained using tissue that was alcohol fixed.
Formalin fixed tissue with all types of antigen retrieval was not reliable.
Rat tissues were fixed in 70% ethanol for 48 hours and then run on a tissue
processor using an alcohol clock.  Morphology was very good, staining was
great and the procedure was very quick.  Antigen retrieval steps are not
needed.  Just be sure to use a secondary that is absorbed against rat.  If
you have any questions, feel free to contact me.

Tim Coskran
Pfizer