Mycological and serological studies were carried out as part of a survey of respiratory disease in Scottish maltworkers. 70% of stained sputum smears from 574 workers showed the presence of higher plant cells and/or myclelia, and the spores of common environmental fungi. Penicillium spp. (90%), Rhizopus stolonifer (48%) and yeasts (53%) were the dominant fungi in 699 sputum cultures, and showed a similar proportional distribution in 327 samples of grain, malt, culms and dusts from fifty-six maltings. 57% of 711 men were serologically positive for fungi, 22% for Aspergillus fumigatus, 20% for A. clavatus, 10% for A. niger, 16% for Cladosporium herbarum and over 3% for Rhizopus stolonifer, 6% of 132 men were positive for Penicillium cyclopium. No precipitating antibodies to antigens from Alternaria tenuis, Aureobasidium pullulans, Candida albicans, Geotrichum candidum, Rhodotorula glutinis or Trichoderma viride were detected in tests of forty sera. Sera from the 5.2% of men with symptoms of extrinsic allergic alveolitis showed increased reactivity to mycelial antigens from Aspergillus clavatus. The fungus was cultured from 21% of maltings, 7% of all environmental samples and from the sputa of 8% of maltworkers.