In two plants producing alkyde and unsaturated polyester resins, the time-weighted average air level during loading of phthalic anhydride (PA) was 6.6 (1.5 to 17.4) mg/m3. In a full workday the level was 0.4 mg/m3. In 60 workers, symptoms of rhinitis and/or conjunctivitis were frequently reported, mostly by heavily exposed workers (69%). Five workers (14%) with PA-associated asthma were found. All were heavily exposed during some period. There was no difference between the exposure groups with regard to total serum level of IgE, IgG, and IgM, nor specific IgE and IgM against PA. There was a significant difference of specific IgG against PA between heavy and low exposure groups (p = 0.01). One worker with asthma had an increased specific IgE level. Subjects with symptoms did not differ from subjects without symptoms in total serum IgE, IgM, IgA, or specific IgE and IgM. However, subjects with rhinoconjunctivitis had lower total IgG than the other workers (p = 0.01). The subjects with asthma had significantly higher values for specific IgG than the asymptomatic subjects (p = 0.005). Four subjects had specific IgG antibodies of subclass 4 (IgG4). Three of these four subjects had asthma, and one had rhinitis. These findings demonstrate that specific IgG is an index of PA exposure and support the hypothesis that specific IgG4 under some circumstances, may be a pathogenetic factor in asthma.