The minimum inhibitory concentrations (mics) of fungi isolated from the air sacs of falcons before (group 1), and during antifungal treatment with amphotericin B nebulisation and oral itraconazole or voriconazole (group 2), or with itraconazole alone (group 3) or voriconazole alone (group 4) were determined. Before treatment, 95 per cent of the isolates, including Aspergillus fumigatus, Aspergillus flavus, Aspergillus niger and Aspergillus terreus, were susceptible to voriconazole at mics up to 0.38 microg/ml, and all the isolates were susceptible at mics up to 1 microg/ml. Before treatment, 21 per cent of the isolates, including A fumigatus (27.6 per cent), A flavus (16.6 per cent), A niger (100 per cent) and A terreus (23 per cent), were resistant (mic > or =1 microg/ml) to itraconazole; 51 per cent of the isolates, including A fumigatus (31 per cent), A flavus (78 per cent), A niger (14 per cent) and A terreus (77 per cent), had mics of over 1 microg/ml to amphotericin B, and after treatment their mics increased significantly. In contrast, there were no significant differences between the mics of voriconazole and itraconazole for the different Aspergillus species before and during treatment with these antifungal agents.