This may not be quite as obvious as it seems on the surface. I searched for "Brahms Polka," and the best information I could come up with was this from the Fiddler's Companion: http://www.ibiblio.org/fiddlers/BRAB_BRAM.htm

BRAHMS POLKA. American, Polka. D Major. Standard tuning. AABB. Indeed, a melodic passage from the classical composer. Source for notated version: Vivian Williams (Seattle) [Phillips]. Phillips (Traditional American Fiddle Tunes - out of print), vol. 2, 1995; pg. 335.

The Washington Traditional Fiddlers' Project has a recording on an out-of-print CD, as has Spirit Fiddle. Doesn't somebody have this in a source that's currently available?

I take it that this is a polka based on a theme from Brahms, and not a polka written by Brahms. Am I correct?
-Joe, intrigued-