A Mass Casualty Incident (MCI) Bag is provided which is a medical supply bag specifically designed to provide emergency supplies to emergency medical technicians in the event of a mass casualty event such as in a public gathering. The MCI bag is designed to allow two medical technicians to treat two or more patients each. The MCI bag is a two-sided, two bag configuration and is designed with identical sets or substantially identical sets of supplies in each bag with some specialty supplies housed in a interconnecting center portion of each bag, where they can be accessed and shared by the two technicians. In one embodiment, the MCI bag is equipped with individual flood lights for each technician to illuminate the accident scene and his patients without the need for hand held flashlights or head lamps. Also provided are a system comprising MCI carts designed to carry multiple MCI bags, and a method of responding to a Mass Casualty Incident, where the method comprises positioning prior to an incident or deploying after an incident, a plurality of MCI carts, each containing a plurality of MCI bags, around the event or incident area.