Debian, multiarch and installing 32-bit libraries on x86-64

I'm running Debian Sid (reasonably up to date). To run a particular 32-bit executable on my x86-64 machine, it appears that I need a 32-bit x86 version of libz.so. The error message from the executable is: error while loading shared libraries: libz.so.1: wrong ELF class: ELFCLASS 64 Given what is happening with Debian multiarch, which is supposed to solve this problem, what's the correct way to install the library at the moment?