Severely Polyester!  The shirt is extremely lightweight, the fabric is shiny, smooth & reminds me of the professional basketball shorts worn years ago. The seams are small, as is typical for this fabric, the navy blue color almost black, and the ordered large (and labeled Large) is the size typically associated with American Medium. The chest measured 40 inches circumference and the shirt was 29 inches long from collar to bottom seam.

That being said, it was the SPF feature that prompted the purchase - cotton shirts cannot be made into adequate sunblock at this time. I have concerns that the replacement XL shirt will be too short as well even as the chest is enlarged a bit. A subsequent review will follow up.