ABSTRACT The microbiology of the air of byres and bruising sheds and of hay, grain and dust from bruising machines was studied in 12 dairy farms in Ayrshire and one in Perthshire. Seven farms (FLD) had a known case of farmer's lung disease and five farms (non-FLD) were free from the disease. Concentrations of mesophilic organisms and of thermotolerant and thermophilic fungi did not vary significantly between the two types of farm but the concentrations of thermophilic actinomycetes and bacteria, notably Micropolyspora faeni, were higher in general on FLD farms. Culture filtrate and mycelial extracts of the most commonly isolated organisms were tested against three groups of sera (11 from patients with farmer's lung disease, 14 from healthy personnel on FLD farms and 13 from personnel without farmer's lung disease on non-FLD farms). Only extracts from a Penicillium sp. and a Streptomyces sp. precipitated with a number of sera, when extracts from the 12 most commonly isolated fungi, from six thermophilic actinomycetes other than M. faeni and Thermoactinomyces vulgaris, and from two thermophilic bacteria were tested. There was no correlation between disease and seropositivity. All sera reacted to at least one of 60 carbol-saline and trichloracetic acid extracts from 30 samples of hay, grain and dust. Although sera from personnel on both FLD and non-FLD farms precipitated with 16% and 19% of these extracts respectively, reactivity to extracts from FLD farms was greater on average than to those from non-FLD farms. When tested by several serological methods against extracts of a type culture of M. faeni and by double diffusion against farmer's lung hay (FLH) and extracts of local isolates of M. faeni, 91% of all clinical cases of farmer's lung were serologically positive but no one test was adequate for determining sensitisation. Fifty-four per cent of sera from FLD and also from non-FLD farms were positive in at least one test. Sixty-nine per cent of seropositive personnel on FLD farms were also symptomatic. The occurrence of symptoms apparently correlated with the higher concentrations of M. faeni encountered on FLD farms.
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