LaTeX macro question

OK, this is not strictly a TeX on Mac OS X question, but maybe some of 
the LaTeX wizards on the list can help me (usually, I'm a ConTeXt 
user). I was trying to write a very simple LaTeX package that would 
help me typeset some parts of my document in a special font: \Mf{blah} 
should be set in a special symbol font. So I had to define the command 
\Mf. I succeeded with some low-level TeX-stuff:

\def\Mf#1{%
  \begingroup
    \fontfamily{newfont}\selectfont#1%
    \endgroup
}

But I had the vague impression that it would be better to do this the 
LaTeX way, i.e., with \newcommand etc. I searched the Companion 2nd 
edition in vain, and no matter what I tried, LaTeX would never return 
to the roman default font after the closing braces. I'm sure it must be 
something embarrassing and obvious; could anybody give me a pointer? 
Thanks so much!

Thomas

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