Guinea pigs could be immunologically sensitised (as shown by the development of antigen-specific homocytotropic antibodies) to toluene diisocyanate by exposing them for 3 h a day for 5 consecutive days to atmospheres containing free chemical. Pulmonary reactions could be elicited in many of the sensitised animals by challenging them with atmospheres containing protein conjugates of the chemical and then measuring changes in respiratory rate. Successful elicitation of pulmonary reactions appeared to depend upon a number of factors, including the quality of the protein conjugate used for the challenge, but possibly also the development of IgE as well as IgG1 antibodies. Antigen-specific homocytotropic antibodies were detected in guinea pigs similarly exposed by inhalation to two non-isocyanate respiratory allergens, trimellitic anhydride and a reactive dye. Although the animals were immunologically sensitised to the chemicals, challenge with atmospheres containing appropriate chemical-protein conjugates failed to stimulate changes in respiratory rate.