Plastic lining. After 6 months of use/washing I took it out of the dryer to see it. The plastic has melted and is no longer waterproof. My other complaint is that the pockets aren't very deep, making it difficult to pull them over the mattress (yes, I know baby bedding can't be loose). If you want a reasonable price this will work as long as you are willing to follow the washing and drying instructions. As a parent of an infant, most baby items are thrown in the same load, which means they have most likely been dried at a higher temperature than recommended. That's more hassle (due to the need to dry on low heat) than the lower price is worth. I ended up buying a Serta mattress topper to replace.
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