Groups of male Swiss-Webster mice were exposed to concentrations of chlorine varying from 0.7 to 38.4 ppm and to concentrations of hydrogen chloride varying from 40 to 943 ppm. The total exposure time to both gases was 10 minutes. Dose-response curves were plotted for both chlorine and hydrogen chloride, using the percentage decrease in respiratory rate during each exposure as the response reflecting sensory irritation of the upper respiratory tract. The results showed chlorine to be 33.0 times more irritating than hydrogen chloride, with 95% confidence limits of 18.6 and 57.1. Guidelines for obtaining a range of acceptable threshold limit values (TLV) based on sensory irritation of the upper respiratory tract are discussed. It was concluded that the current TLV of 1 ppm for chlorine is the upper acceptable limit, and that the established TLV of 5 ppm for hydrogen chloride lies at the lower limit of the predicted range. The mechanism of chlorine’s and hydrogen chloride’s sensory irritation may be explained by their reaction with various functional groups in the membranes of the trigeminal nerve endings lining the nasal mucosa.