Sera from all housed adult cattle in twelve herds were examined for precipitating antibody to Micropolyspora faeni at the beginning of winter and approximately 5 months later in the spring; during this time they had been fed hay. Six of these herds were owned by farmers with farmer's lung. The average incidence of precipitins was 33% on the first occasion and 47% on the second occasion. After the winter there was a statistically significant increase in the number of animals with precipitins in five herds. A positive correlation was shown between the incidence of precipitins and age.

None of seventy-five animals on a farm feeding good hay and only one of fifty-eight cattle fed silage had precipitins after the winter. Whereas, on a farm where the farmer complained of a recurrence of the symptoms of farmer's lung when feeding hay, the incidence of precipitins in his cows increased dramatically from 9% to 76%. A farm with a respiratory disease problem in the cattle, diagnosed as bovine farmer's lung, had 83% of its animals positive.

The significance of precipitins and the problem of diagnosing farmer's lung in cattle are discussed.