using procmail on stdin

Is there a way to use procmail or any other commands piped together
to context grep through a large mail file coming from stdin?  I
have some old mail files that are from before I was using procmail
to sort out my mail that are 25M or so that vi croaks on.  I have
used split to chop them up but it would be so much nicer if I could
just grep out the entire message based on "^From: blah(_at_)blah" or
"^Subject:  blah blah" that I wanted to keep and throw the rest
away.

Thanks,
Terry Todd