Farm and sawmill workers are exposed to high levels of allergenic fungi, such as Alternaria alternata, which are associated with respiratory diseases and asthma. The aim of this study was to measure the concentration of Alt a 1, a major allergen of A. alternata, in indoor dust samples collected in poultry farms and a sawmill using a monoclonal antibody-based enzyme immunoassay. A total of 45 dust samples were collected in poultry farms (30) and the sawmill (15) in Zagreb County (Croatia). The Alt a 1 allergen was detected in all dust samples (100%) collected in three poultry farms. The levels of Alt a 1 were in the range of 0.1-14 microg/g, and the median value was 0.37 microg/g. About 86% of dust samples contained Alt a 1 in the range of 0.1-1.0 microg/g. In the sawmill, no detectable level of Alt a 1 was found (limit of detection =0.12 microg/g). This study has shown that occupational exposure to Alt a 1 allergen in poultry farms deserves monitoring.