Mass. Gen. Laws ch. 146, § 84: License requirement

Section 84. No person shall engage in the business of a master pipefitter, work as a journeyman or apprentice pipefitter, work as a refrigeration technician or work on a fire protection sprinkler system as a fire protection sprinkler system contractor or as a sprinkler fitter or apprentice sprinkler fitter, unless that person is lawfully licensed under section 85 or section 85A.