Will reference letters from a researcher who does not have a Ph.D be valid? I'm applying to Ph.D. programs in the biomedical field in the US but I have a question regarding reference letters:
Can I request a reference letter from a co-worker that has no Ph.D. (instead has been working in a research institute for many years)  or a professor from my undergrad university (does not have a Ph.D. but has years of experience) and still be valid? Or should I request a reference letter just to researchers that have a Ph.D.?
You can, of course, request a reference letter from anyone. However, the point of having a reference letter is to speak to your ability to succeed in a research program. Because a PhD is a marker of having succeeded in the same program they're recommending you for, it is highly recommended that you get a letter from someone with a PhD.
If these researchers know you well, then their letters can be valuable, but you should make sure to get a letter from someone with a PhD, if you can.