# %& magic line has no effect

I am using the %&format magic line in a plain TeX file. On older versions of TeX the result produces the expected results. On TeXLive 2009 (under Gentoo), however, this line has no effect. When I use 'tex -fmt format.fmt file.tex' the expected font substitutions nevertheless work. Has the magic file feature to be enabled in some config file? Many thanks.

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