According to Wilson Sporting Goods and Penn Racquet Sports Company, their tennis balls are made of rubber and latex, and the felt covering the ball consists of wool and nylon. The dyes used to color the felt are non-toxic. I currently do not have information from other manufacturers, so I cannot say anything about the safety of their balls. If you come across such information, please let me know. Latex is probably the most harmful of these materials, since some dogs could be allergic to it. Latex is generally used in rubber gloves, for example, and some humans are allergic to it too.