A limited study has been made of the toxic effects of hydrogen chloride, representing the major toxic component of the pyrolysis products of poly-vinyl chloride, in the presence of carbon monoxide generated by the combustion of a wood-based material (hardboard). In the presence of the mixed gases, at concentrations within the range of approximately 2000-20,000 mg/m^3, deaths among rats and guinea pigs exposed for 30 minutes were primarily due to carbon monoxide poisoning, but the hydrogen chloride was found to enhance the response to the carbon monoxide. However this enhancement occurred mainly at concentrations of hydrogen chloride which could be lethal when present alone. There appears to be sufficient evidence available to indicate that the presence of hydrogen chloride at lethal concentrations in fire gases containing lethal concentrations of carbon monoxide could marginally increase the fatalities in fires. It is probable, however, that the most important effects of the presence of hydrogen chloride will accompany sub-lethal exposures, firstly because the highly irritant nature of the gas may result in more people being prevented from using escape routes in the early stages of fires in buildings by concentrations of fire gases and smoke which may be otherwise relatively harmless at the time and, secondly, because survivors may suffer long term, even permanent, injury from high concentrations of this gas. These are the aspects which appear most to require further study.