I thought it might be nice to publish instructions about how to expose and develop leftover rolls of Kodachrome as black-and-white film, but after looking in to it, I don't recommend it. The reason is that Kodachrome has rem-jet anti-halation backing like motion picture negative films. You can expose Kodachrome at about half its ISO and develop in any continuous-tone developer such as D-76 and get a usable (even good) black-and-white negative image (the film can also be processed with reversal processes to achieve a monochrome positive), but the problem is that you still have to remove the rem-jet backing, which sounds like a fussy, messy, and labor-intensive project to me (it involves soaking the film in a borax solution and wiping away the rem-jet by hand, with sponges. According to those who've done it, you'll use about four sponges per roll, because they foul quickly). I've been a darkroom tech in the past and I wouldn't want to have to do it. Whatever money you save by using the old film would be more than offset by the labor involved in processing it.
So here's my considered recommendation as to what to do with leftover unexposed Kodachrome: throw it away (or take it to a lab that does silver recovery), and if you want to shoot some old-fashioned looking B&W, buy some fresh Plus-X and pull it. Sometimes the simple solution is the best solution.
The only reason to process old Kodachrome would be if it's already been exposed at some point in the past and you have reason to believe the information in the images might be valuable (a deceased relative, top-secret spy stuff, etc.). In that case I would recommend finding a good custom lab willing to take on the task (try a lab that handles motion-picture film), rather than attempting it yourself.
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Original contents copyright 2010 by Michael C. Johnston and/or the bylined author. All Rights Reserved.
Featured Comment by Paul Glover: "There are a couple of places which will develop Kodachrome as B&W, Blue Moon Camera and Machine being one of them if I recall correctly. [I have not confirmed this, as Blue Moon is closed for New Year's weekend. —Ed.]
"By the time you consider the relatively high cost of doing this vs. buying some new film and developing it yourself the only reason to bother at all would be to get at the images on already exposed film.
"As others have mentioned, I'd rather support those who are selling new film so they're more likely to continue making it, which is the same reason why I don't go for cheap expired film deals on auction sites."