How do I get a smell out of bath towels?

You might try washing them in laundry soap with half a cup of vinegar

Wash them in baking soda. Totally removes the smell.

I'd re-wash them in hot water with soap and bleach and vinegar. Then be sure you're drying them long enough in the dryer. Are you in a super humid climate? Or do you need to install an exhaust fan in the bathroom?

Add washing soda, bleach (a small amount), white vinagar or oxyclean to your wash. Then rewash the towels and dry thoroughly. It is likely the towels were not dried enough or became sour after using.

Sometimes a washer can cause the moldy odor. If you have a front load washer it might need a good cleaning of the gasket and tub. Wipe out the gasket, clean the glass door and run a hot cycle with your usual laundry soap adding vinegar or bleach. (I do this when washing rags.)

I AGREE WASHING MACHINES NEED TO BE CLEANED OUT REGULARLY ESP. UNDER THE GASKET FOR FRONT LOADERS...…...OXY CLEAN ODOR BLASTERS IS VERY GOOD FOR STUBORN SMELLS...HAS A SCENT ADDED DON'T KNOW IF THAY MAKE A UNSCENTED ONE...YOU MIGHT WANT TO COOL OFF 'ANYTHING' THAT COMES OUT OF THE DRYER BEFORE FOLDING THEM AND PUTTING THEM AWAY, IF THE MOISTURE STAYES IN THEM FROM THE DRYER HEAT SMELLS WILL NOT GO AWAY, I HAVE HAD THIS HAPPEN TO ME AND ALSO SAW THIS ON A SITE ABOUT ODORS IN CLOTHES SOMEWHERE ON THE COMPUTER....I really do not like to use bleach for everything can't stand the smell, I make my own powder laundry soap/fabric softener lasts a lot longer and I can adjust the smell of the soap and fabric softener what they have on the market smells tooooo strong...I can always tell when my neighbor is washing clothes you can smell GAIN ALL OVER THE NEIGHBORHOOD! GOOD LUCK

Add some baking soda to your wash load, works great

Bleach does nothing but weaken the fibers. If you have towels in pretty colors, bleach ruins them. Just add vinegar in the wash and rinse cup and baking soda. I add 2 heaping tablespoons per load, but if they are musty, I would add a 1/4 of soda per load. Keep the clean towels separate from the not yet laundered towels. Wipe out the linen closet with white vinegar and air out before returning items.