Spread 1 1/2 tsp. mayonnaise on 1 side of each bread slice.
Heat a large nonstick skillet over medium heat. Place 2 bread slices, mayonnaise side down, in skillet; top each with 2 cheese slices and 1 bread slice, mayonnaise side up. Cook 3 minutes on each side or until golden brown. Repeat with remaining bread slices and cheese.