The wearing of dust respirators has been advised for the prevention of dust-induced pulmonary disease in agricultural workers (Medical Research Council 1968). The effectiveness of such respirators in the prevention of recurrance of farmer's lung has been questioned (Medical research Council 1968). We report here the results of a ‘Provocative trial’ carried out on a farmer who suffers from farmer's lung, together with findings from other farmers who have been using respirators for some months.