The occupational effect of carbon monoxide on blood carboxyhemoglobin levels (COHb) from smoke inhalation was studied in 51 fire fighters. Nonsmoking fire fighters had a baseline mean blood COHb saturation higher (P<.005) than non-smoking controls, while there was no difference in the blood COHb between smoking fire fighters and smoking control subjects. The blood COHb was repeated in 16 fire fighters after three building fires and there was a significant increase (P<.001) in the mean blood COHb level over the baseline values. Fire fighters should avoid cigarette smoking after a fire to prevent further increases in blood COHb levels. The use of the self-contained air breathing apparatus is protective against undue exposure to carbon monoxide during fire fighting.