A 64-bit Mac Chimera daily build is now available for Mac OS 10.6. It will not run on Mac OS 10.5. http://www.cgl.ucsf.edu/chimera/download.html#daily If you work with EM maps larger than 1 Gbyte or molecular models of several hundred thousand atoms the larger addressable memory in the 64-bit version will help avoid out-of-memory errors and crashes. The 32-bit Mac Chimera versions can only address 4 Gbytes of memory and because libraries, stack and other uses of memory the practical working data size limit for 32-bit versions of Chimera is about 1 Gbyte. You should have more than 4 Gbytes of memory installed in order to benefit from the 64-bit version. The 64-bit Mac Chimera is in early testing so will have more bugs than usual. I've tested the 64-bit Mac Chimera running Gaussian filtering on a 2048 by 2048 by 800 map requiring about 3.2 Gbytes for the floating point result and 0.8 Gbytes for the original map. This worked correctly (took about 3 minutes on a machine with 6 Gbytes of memory) but failed with 32-bit Mac Chimera. We are also working on a 64-bit Windows Chimera that will work with Windows 7. This may be available within a month if no serious obstacles are encountered. A 64-bit Linux Chimera has been available for a few years. Tom