INTRODUCTION This study addresses itself to the biological relevance of SO2 inhaled at low concentrations. Sulfur dioxide is an interesting product of fuel combustion@ which appears with ubiquitous distribution in urban air. For the past 30 years sulfur dioxide has been suspected to be the main factor of injury in men exposed to air pollution. The attention devoted to this pollutant@ however@ has filed to provide up to date@ convincing evidence to support this opinion. Among several hypotheses and inferences@ originating from bench experiments and from epidemiological intelligence@ one possibility has emerged for the role played by sulfur dioxide in favoring pulmonary infections. The impediment imposed on ciliary clearance@ verified at high concentration of sulfur dioxide inhaled for short duration@ has been suggested as a relevant mechanism for this type of injury. An additional mechanism of damage@ realized independently from the ciliary inhibition@ has also been postulated. This is an unidentified infection of facilitating mechanism@ perhaps originating from a locus minoris resistentiae opened by sulfur dioxide upon the respiratory surfaces. This study was conducted between April@ 1966 and April 1969. During the early part of the first year (April-December@ 1966)@ the exposure chambers and the monitoring procedures were assembled and tested. During the same period@ the biological indicators were calibrated for accuracy and reproducibility and the related techniques were standardized.