Can you microwave a Ziploc bag?

If you are only warming something up to below boiling point it would work. Over that the bag will expand and open the zip or burst. Polythene is not suitable for high temperatures either, it deforms, melts and can transfer a taste to food. Generally I would not advise it, There are special microwave bags available.

Have you heard of bisphonol A(BPA)??? It is a chemical compound that is in plastics including the plastic coating on the inside of most cans. Remember when you emptied a can and the inside was just more tin, but today's cans have a smooth white lining. This white lining is plastic and contains BPA's. If you microwave food in any plastic container, including a zip lock bag, BPA's seep into your food...For more information just google BPA.