A computer model for simulating the early fire growth in an enclosure is described. The enclosure is assumed to be divided into distinct homogeneous zones. The rate of heat release is defined for the fire and the conservation equations are solved for zones, yielding the temperature and radiant heat conditions in the fire room. Results of the model are compared with measured data of the burning of wood and plastics cribs in an enclosure, showing a good agreement between measured and calculated results. This work was financially supported by the Swedish Fire Research Board.