An outbreak of hypersensitivity pneumonitis in a textile plant was attributed on epidemiological grounds to a chilled water air-conditioning system where a slime was growing in the chilled water sump and on demister vanes. All of the patients and about 80% of their asymptomatic co-workers had strongly positive precipitin tests to extracts of the slime. An adoptation of a radioiodinated staphylococcal protein A solid-phase radioimmunoassay was applied to monitor antigen and specific antibody. To control the outbreak, a variety of cleaning and water treatment measures were taken between 1977 and 1979 to reduce the amount of antigen in the water and in the air. The amount of slime in the water was greatly reduced though the antigen content per gram of slime did not change. Airborne antigen in the affected work areas decreased progressively.