The toxic hazard from cyanide produced in fires was assessed in casualties and in firemen exposed to the fire atmosphere. The levels of cyanide and its principal metabolite, thiocyanate, were measured in blood samples from fireman, non-fetal and fatal casualties, and from controls. Although firemen did not differ significantly from controls, casualities showed significant elevation of blood-cyanide, and in a small proportion of fatalities blood-cyanide reached toxic levels.