- Machine Shop & Tool Repair Service, Millbury
- Machine Shops Experimental, Millbury
- Machinery, Supplies & Equipment - Business Production Related, Millbury
- Machinery, Supplies & Equipment - Business Supplies Related, Millbury
- Machinery, Supplies & Equipment - Construction & Contractor Related, Millbury
- Magazine & Newspaper Stands, Millbury - Dealers & Distributors
- Major Appliances, Millbury - Household & Kitchen
- Malls & Shopping Centers, Millbury
- Management Consultants & Services, Millbury
- Manicurists & Pedicurists, Millbury
- Manufacturers Agents & Representatives, Millbury
- Manufacturing, Millbury
- Manufacturing Products, Millbury
- Marble Contractors, Millbury - Installation, Cleaning & Removal Services
- Marriage & Family Counselors, Millbury
- Martial Arts & Self Defense Schools, Millbury
- Masonry Materials & Supplies, Millbury
- Medical & Dental Management, Millbury
- Medical Management & Business Administration Service, Millbury
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- Medical Supplies & Equipment, Millbury
- Men's Clothing Stores, Millbury - Clothes & Accessories
- Merchandise Sales Events, Millbury
- Metal Heat Treating, Millbury
- Mexican Restaurants, Millbury
- Miscellaneous Household Item Repair, Millbury
- Miscellaneous Government, Millbury
- Miscellaneous Manufacturing, Millbury
- Miscellaneous Personal Services, Millbury
- Miscellaneous Professional, Millbury
- Mortgage Bankers & Correspondents, Millbury
- Motor Vehicle, Millbury
- Movie Theaters, Millbury
- Muffler Shop & Exhaust Systems, Millbury
- Mulch, Peat & Moss Products, Millbury
- Music, Millbury
- Musical Instrument Rental, Millbury - Band & Orchestra Instruments
- Musical Instruments & Equipment Stores, Millbury
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