Latex bladder.

Size S used for speed bags measuring 8" x 5" (item numbers 220801 & double end bags-need replacement bladder 4230); Size L used for medium & large speed bags. Measuring 9" x 6", 10" x 7" and 11" x 8" bags (item numbers 220901, 221001, 4215, 4241, 4242, P00000341, P00000774).