Humidifier lung is a form of exogenous-allergic alveolitis caused by microbial growth in humidifiers and air conditioners. It was the aim of the present study to employ and test the ELISA method as an alternative to antibody determination. 134 employees in a large printhouse equipped with air conditioning plant were examined by us. Specific IgG antibodies against contaminated humidifier fluid were determined by means of a solid-phase radioimmunoassay (protein A RAST) that we had developed further. Alternatively we examined a commercially available ELISA method (Pharmacia IgG-RAST 40; enzyme: beta-galactosidase) and an assay based on protein A peroxidase. The influence of different test conditions was studied. All the methods examined proved suitable for determining the specific IgG antibodies. The commercial beta-galactosidase assay could be adapted to application on microtitre plates in a slightly modified form. In the peroxidase assay it is recommended to use very low serum and enzyme concentrations on account of its high sensitivity. Examination of all the 134 serum samples yielded a high correlation between the results of these two non-radioactive methods and those obtained with the protein-A RAST.