What's the best way to remove permanent marker from concrete floor?

Acetone?

Alcohol. but, use it sparingly and stay within the marks of the permanent marker. the alcohol will fade or leave a light mark on the concrete. you will have to use some strength to rub the marks, so get the bengay ready.

lacquer thinner,although the crete is porous so it might be tough to get it all out.

My daughter used that Magic Eraser by Mr. Clean to remove magic marker from the walls, cabinets, refrigerator, etc. It would probably work on floors, too. (One of my grandsons loves to draw…)

Look here :-/

http://www.wes.army.mil/REMR/pdf/cs/mr-4-4.pdf

Muratic Acid. Just a suggestion. I’ve never tried it but i know it etches concrete. It might even be used in the prep for the stain.
I have used floor stripper to remove paint form concrete. Never had a need to try it on magic marker. I used a stripper made by ZAP. I got it Home Depot.
I’d love to know if you get something to work.

For all who are still following, I think we found a great solution. Bleach seems to work like a charm when you want to remove permanent marker from concrete. Our house was built about 15 years ago, so I imagine the marker has been there for that long and was deep within multiple layers since concrete is porous. We tried bleach this morning and there is still a very slight discoloration where the marker was, but it’s barely noticeable and I imagine once we stain, it will be gone. Hope that helps anyone searching for an answer to permanent marker on concrete!