The immunologic reaction of workers in different waste treatment facilities to inhalation of mold antigens was studied by determination of specific IgG antibody levels and compared to exposures. Antigens were prepared from cultures of fungi which were dominant in dust samples at these workplaces. In 520 waste workers, 32 patients with symptoms of hypersensitivity pneumonitis (HP), and 98 control subjects, serum concentrations of specific IgG antibodies to antigens of molds (Aspergillus and Penicillium species) were determined by an indirect immunofluorescence test (IIFT). The highest antibody levels were found in workers in compost plants, followed by employees of garbage sorting facilities. Lower levels were determined in workers at landfills and lowest in collectors of household biowaste. This order reflects the ranking according to the exposure measurements. About 5% of workers had elevated antibody levels, but none complained about typical symptoms of HP. The IIFT proved to be a very sensitive method of antibody detection since even IgG levels of persons subjected to environmental exposure alone (control) could be measured. High exposure to organic dust at workplaces with waste handling is correlated with elevated mold-specific IgG antibody levels.