- Machine Shop, Port Jefferson
- Machine Shop & Tool Repair Service, Port Jefferson
- Machinery, Supplies & Equipment - Business Supplies Related, Port Jefferson
- Machinery, Supplies & Equipment - Construction & Contractor Related, Port Jefferson
- Magicians & Hypnotist Entertainers, Port Jefferson
- Mail Delivery Service, Port Jefferson
- Major Appliances, Port Jefferson - Household & Kitchen
- Management Consultants & Services, Port Jefferson
- Manicurists & Pedicurists, Port Jefferson
- Manufacturers Agents & Representatives, Port Jefferson
- Manufacturers Representative, Port Jefferson - Reps, Independent Sales Representative, Sales Agent, etc
- Manufacturing, Port Jefferson
- Marinas & Marina Services, Port Jefferson
- Marine Hardware, Port Jefferson
- Marine Products And Services, Port Jefferson
- Marine Repair & Service Stations, Port Jefferson
- Marine Supplies & Equipment, Port Jefferson - Boat Supplies
- Marine Supplies & Equipment Repair, Port Jefferson - Boat Supplies & Equipment
- Market Research & Analysis , Port Jefferson - Market Analysis, Business & Economic Research
- Marriage & Family Counselors, Port Jefferson
- Masonry Contractors, Port Jefferson - Stone Mason, Brick Mason - Install & Repair
- Massage Therapists & Professional, Port Jefferson
- Massage Therapy, Port Jefferson
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- Mastectomy Apparel, Forms, Prostheses & Supplies, Port Jefferson
- Medical & Health Service Organizations, Port Jefferson
- Medical Diagnostic Clinics, Port Jefferson - Clinical Labs & Centers
- Medical Supplies & Equipment, Port Jefferson
- Membership Sports & Recreation Clubs, Port Jefferson
- Mental Health Clinics, Port Jefferson - Treatment Centers
- Mental Health Specialists, Port Jefferson - Psychologist, Psychiatrist, Psychotherapist, etc
- Methodist Church, Port Jefferson
- Mexican Restaurants, Port Jefferson
- Ministries Church, Port Jefferson
- Miscellaneous Government, Port Jefferson
- Miscellaneous Manufacturing, Port Jefferson
- Miscellaneous Personal Services, Port Jefferson
- Miscellaneous Photographers, Port Jefferson
- Miscellaneous Professional, Port Jefferson
- Miscellaneous Toys & Supplies, Port Jefferson
- Mortgage Bankers & Correspondents, Port Jefferson
- Motor Vehicle, Port Jefferson
- Motorcycle Parts, Accessories & Dealers, Port Jefferson
- Multimedia Publishers, Port Jefferson - Multimedia Services & Systems
- Music, Port Jefferson
- Musical Instruments & Equipment Stores, Port Jefferson
- Musical Instruments Repair, Sales & Service, Port Jefferson
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