The Dunlop Grand Prix (Extra Duty) Tennis Balls (Case) offers the ultimate in performance and durability. A special mix of fourteen ingredients make-up the secret rubber core formula which is combined with the finest woven felt to produce a ball for outstanding performance and feel. The Grand Prix Hard Court balls feature the MaxGlo felt. The MaxGlo felt is 14% brighter than regular cloth, allowing you to see the ball earlier to give you more time time for shot preparation. The Hard Court version is made specifically to withstand the abrasion from hard courts. Also available is the Dunlop Grand Prix Regular Duty tennis ball that can be used on clay or indoor courts. The Dunlop Grand Prix is UTSA and ITF approved for tournament and league play.