There is substantial evidence that workers handling grain develop allergic respiratory symptoms. Microbiological contaminants are likely to be a significant contributing factor. Worker's exposure to microorganisms contaminating grain dust in the UK was therefore examined. Aerobiological studies were made when grain was being handled on farms and also during bulk handling of grain in dockside terminals. A quantitative and qualitative microbiological examination of the airborne grain dust was carried out. Samples of airborne grain dust were collected and viable bacteria, fungi and actinomycetes were grown, isolated and identified. It was found that workers handling grain or working close to grain at farms and docks were frequently exposed to more than 1 million bacteria and fungi per m³ air, and that airborne bacteria and fungi exceeded 10⁴; per m³ air in all areas sampled. The qualitative examination of the samples showed that the predominant microorganisms present differed between freshly harvested grain and stored grain, but not between different types of grain.