- Machine Shop, Catonsville
- Machine Tools , Catonsville
- Machinery, Supplies & Equipment - Business Production Related, Catonsville
- Machinery, Supplies & Equipment - Business Supplies Related, Catonsville
- Machinery, Supplies & Equipment - Construction & Contractor Related, Catonsville
- Mail Delivery Service, Catonsville
- Mailing, Delivery & Courier Service, Catonsville
- Maintenance, Cleaning & Sanitation, Catonsville
- Major Appliances, Catonsville - Household & Kitchen
- Make Up & Cosmetics Application, Catonsville
- Malls & Shopping Centers, Catonsville
- Management Consultants & Services, Catonsville
- Manicurists & Pedicurists, Catonsville
- Manufacturing, Catonsville
- Market Research & Analysis , Catonsville - Market Analysis, Business & Economic Research
- Marriage & Family Counselors, Catonsville
- Martial Arts & Self Defense Schools, Catonsville
- Masonry Contractors, Catonsville - Stone Mason, Brick Mason - Install & Repair
- Massage Therapists & Professional, Catonsville
- Massage Therapy, Catonsville
- Meat Market, Wholesale & Products, Catonsville
- Meat Packers, Catonsville
- Mechanical Contractors, Catonsville - Install, Service & Repair
- Mechanical Engineer, Catonsville - Mechanical Engineering Service
- Medical & Dental Management, Catonsville
- Medical Billing Services, Catonsville
- Medical Business Management Services, Catonsville
- Medical Diagnostic Clinics, Catonsville - Clinical Labs & Centers
- Medical Equipment Manufacturers, Catonsville
- Medical Management & Business Administration Service, Catonsville
- Medical Research, Catonsville
- Medical Research & Development, Catonsville - Medical Information & Research
- Medical Supplies & Equipment, Catonsville
- Membership Sports & Recreation Clubs, Catonsville
- Mental & Developmental Information & Referral Services, Catonsville
- Mental Health Specialists, Catonsville - Psychologist, Psychiatrist, Psychotherapist, etc
- Mental Retardation & Developmentally Disabled Services, Catonsville
- Methodist Church, Catonsville
- Mexican Restaurants, Catonsville
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- Middle Eastern Restaurants, Catonsville
- Midwives, Catonsville
- Mini & Self Storage, Catonsville
- Miscellaneous Business Credit Institutions & Services, Catonsville
- Miscellaneous Business Product Repair, Catonsville
- Miscellaneous Household Item Repair, Catonsville
- Miscellaneous Retail Stores, Catonsville
- Miscellaneous Business Services, Catonsville
- Miscellaneous Commercial & Industrial Supplies & Equipment Rental & Leasing, Catonsville
- Miscellaneous Government, Catonsville
- Miscellaneous Manufacturing, Catonsville
- Miscellaneous Personal Services, Catonsville
- Miscellaneous Photographers, Catonsville
- Miscellaneous Professional, Catonsville
- Modular & Mobile Homes For Sale, Catonsville - Manufactured Homes Sale & Service
- Money Transfer Service, Catonsville
- Mortgage Bankers & Correspondents, Catonsville
- Mortgage Companies, Catonsville
- Mortgage Contracts & Services, Catonsville
- Mortgages & Loans, Catonsville
- Mortgages Commercial & Industrial, Catonsville
- Motion Picture Pre & Post Production Services, Catonsville
- Motor Vehicle, Catonsville
- Motor Vehicle Rental & Leasing, Catonsville
- Moving & Storage Consultants, Catonsville
- Moving & Storage Supplies & Equipment, Catonsville
- Moving Companies, Catonsville
- Moving Services - Labor & Material, Catonsville
- Moving Supplies & Equipment Rental, Catonsville
- Moving Truck Rental , Catonsville
- Muffler Shop & Exhaust Systems, Catonsville
- Multimedia Publishers, Catonsville - Multimedia Services & Systems
- Music, Catonsville
- Music Lessons, Catonsville
- Music Lessons - Instrumental, Catonsville
- Musical Instrument Rental, Catonsville - Band & Orchestra Instruments
- Musical Instruments & Equipment Stores, Catonsville
- Musical Instruments Repair, Sales & Service, Catonsville
- Musical Recording Services, Catonsville
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