Here's a quick example of an AmSTeX file including some Cyrillic text.
\documentstyle{amsppt}
\input cyracc.def
\catcode`\@=11
\font@\tencyr=wncyr10
\font@\eightcyr=wncyr8
\catcode`\@=13
\addto\tenpoint{\def\cyr{\tencyr\cyracc}}
\addto\eightpoint{\def\cyr{\eightcyr\cyracc}}
\document
English text \dots {\cyr chto-to po-russki} \dots more English text.
\enddocument
Last updated 21 February 2005