Mace, or pepper spray, is legal in the United States, but in some states, including Massachusetts, use is restricted. You are required to carry a license class B Restricted FID (Firearms Indentification) card, M.G.L. chapter 140, section 29B, because self-defense spray is considered ammunition.
Print the license application or get it from your local law enforcement department. Be prepared to provide extensive personal information, your birth certificate, fingerprints, and two references, then call your local police department, in case you need an appointment to drop it off with the $25 fee. You will have to wait a few weeks for license approval. If approved you must purchase the spray from Licensed Firearms Dealers. Massachusetts does not allow mail order of pepper spray. The license is good for 6 years, and normally the Mass Department of Fish and Game, sends out renewals 90 days before the license is due to expire. Don't let it lapse, Massachusetts imposes strict fines, M.G.L. chapter 140, section 139, for unlicensed possession.
If you are traveling by air, FAA regulations allow one four-ounce container or smaller of pepper spray in checked baggage. For more information see our law about guns page.