Abstract: A fire fighting system comprising a fixed fire fighting system with a plurality of spray heads (15) as well as means (10, 12, 14A, 14B, 14C) for leading a fire fighting medium from a fire fighting medium source (11) to the spray heads (15) as well as at least one pump means (10) between the fire fighting medium source (11) and the spray heads (15), which system further comprises spray heads (5) arranged in a mobile unit (1), such as a vehicle, a carriage connectable to a vehicle, a train or a train car, and means (2, 4) for leading a fire fighting medium to the spray heads (5). The system comprises means (7, 7?, 8, 9) for connecting the spray heads (5) of the mobile unit (1) to the circuit of the fixed fire fighting system.