This study was carried out to find the prevalence of precipitin reactions in the sera of 200 North American asthmatic subjects. Precipitins were detected by the double diffusion technique using different extracts of Aspergillus fumigatus, including a reference ‘home produced’ extract and five commercial extracts from three different suppliers. In addition, antigenicity of these extracts was assessed by crossed immunoelectrophoresis (XIE). Of the sera, 13.5% reacted to the reference extract and from 2.5 to 12% to the different commercial extracts; 22.5% of the sera reacted to at least one extract. No one serum reacted to all the extracts. Two of fifty-one (4%) non-atopic patients with a negative immediate prick test to A. fumigatus, six of eighty-seven (6.9%) atopic patients with a negative immediate reaction to A. fumigatus, and thirty-seven of sixty two (59%) atopic patients with a positive immediate reaction to A. fumigatus had precipitins to at least one of the extracts used, the skin tests being performed using the A. fumigatus reference extract. The prevalence of precipitin reactions bore a strong correlation with the antigenicity of the extracts by XIE. The same reference extract was also used for specific IgE measurements (Brompton extract, Malo & Paquin, 1979). It was found that patients with precipitins had significantly (P < 0.001) higher specific and total IgE values than patients without precipitins. In the group of patients with positive skin test, those with precipitins had significantly (P<0.05) higher specific IgE values than those without. The authors conclude that different extracts of A, fumigatus should be used to assess the presence of precipitins. The antigenicity of these extracts should also be assayed.