Tests were conducted in an intermediate-scale fire tunnel and a large-scale fire gallery to determine the effects of fires on the ventilation flow. Results of these tests were applied to a full- scale mine fire conducted in the Bruceton Experimental Mine with very good agreement. The analysis predicts a limit fire size, dependent only upon the passageway dimensions, below which ventilation reductions are significant and independent of fire size. For larger fires a simplified theory, based upon the measurement of co2 concentrations, was found to be valid. Continuous monitoring of co2 can also provide significant information relative to the fire hazards that result.