Antibody activity in the major classes and IgG subclasses against antigens in factory humidifier water was quantified by enzyme immunoassay (EIA) in 88 subjects who were exposed at work to the output from these contaminated humidifiers. Those with work-related symptoms had significantly higher mean titres than those who were symptom free, although values overlapped. The individuals with the highest IgG antibody titres also had the highest titres of IgM and IgA antibody, and these parameters did not discriminate between those with and without symptoms any better than the IgG titre. This was also true for the IgG subclasses where activity was predominantly measured in IgG 1. Quantifying the IgG antibody allowed us to demonstrate a significant correlation with years of work exposure (P<0.001). There was no significant association between antibody and cigarette smoking, as assessed by smoking history and confirmed objectively by serum cotinine levels. There was a significant correlation with total IgG level (P<0.001) suggesting that a non-specific immune enhancement may accompany the specific response. The antibody titres were followed up to 3 years after modification of the humidification systems, and during this time symptoms resolved and the antilunK levels progressively fell to undetectable levels. The EIA was adapted to measure antigen at nanogram levels thus providing a rapid test for screening of humidifier water as a technique that may help identify the nature of the antigens involved.