When exposed to an amount of contaminated humidifier water roughly equivalent to that inhaled over an 8-hour period at their work place, four out of six subjects developed symptoms of humidifier fever. Two non-exposed subjects failed to react to the same challenge. Characteristic lung function, temperature and leucocyte changes were recorded; however, a fall in gas transfer previously reported was not seen. That the reaction was immunologically mediated and not due to endotoxin activity was shown by a negative pyrogen response in rabbits inoculated intravenously with concentrated humidifier water. The nature of the immune response has not as yet been evaluated but it does not reside with the ability of humidifier fever antigens to activate complement. Skin testing produced an immediate weal and flare in the four subjects with precipitins and may reflect the presence of short-term anaphylactic IgG antibody.