Smartwool. Enjoy four seasons of great comfort, durability, and fit with our Men's Merino 150 Baselayer Long Sleeve. This year-round performer features our lightest-weight Merino jersey fabric with a nylon core to increase durability while still keeping 100% soft Merino wool next to skin. Its shoulder and seam construction is designed to minimize chafing while wearing a pack, and new select colorways feature a more responsible plant-based-dye technique. Gear up to go farther on every pursuit with this base layer.