Doctoring the Baseball
No player is permitted to intentionally damage, deface or discolor the baseball by rubbing it with any type of foreign item or substance, including dirt or saliva. Failure to follow this rule will result in an ejection and an automatic 10-game suspension.
The pitcher is allowed to rub the ball between his bare hands but cannot spit on the ball, his hands or his glove. Also, pitchers are not allowed to rub the ball on their clothes, glove or other body parts besides their hands, nor are they permitted to pitch with an attachment such as a bandage, tape or bracelet on either hand or wrist.
While in contact with the pitching rubber, the pitcher is not allowed to touch his mouth or lips at all. He can touch his mouth or lips when in the 18-foot circle surrounding the pitching rubber, but he is not permitted to then touch the baseball or the pitching rubber without first wiping his pitching hand dry. The pitcher shall be issued a warning the first time he violates this rule and then the umpire shall call an automatic ball for each subsequent violation. The pitcher may be allowed to blow on his pitching hand in a game played in cold weather, provided both managers agree to that exception prior to the start of the game.
History of the rule
Pitches utilizing foreign substances, such as the spitball, were outlawed in 1920, but teams were allowed to designate up to two pitchers who could legally use the spitball during the 1920 season. Following that campaign, Major League Baseball designated 17 pitchers as legal spitballers who were permitted to use the spitball for the remainder of their careers. The spitball hasn't been legally used since Burleigh Grimes retired in 1934.