A new method of retting flax recently developed to suit the United Kingdom climate has allowed the reintroduction of flax growing to this country. The weed killer glyphosate is sprayed on the crop which then rets before harvesting six weeks later. The acute bronchoconstrictor responses of 11 normal subjects to dust from dew retted and from pre-harvest retted flaxes were compared in a double blind crossover fashion. There were no significant differences in the dust levels nor in the size of the dust particles in the experimental dust room. The decreases in pulmonary function after six hours of dust inhalation were significantly larger after pre-harvest retted flax dust than after dew retted flax dust (delta FEV1, -0.21 and -0.40 1; delta MEF50, -0.72 and -1.211/s; delta sGaw (specific airway conductance), -0.17 and -0.65 kPa/s for dew retted and pre-harvest retted respectively). The subjects also reported more symptoms after inhaling pre-harvest retted flax dust. It is concluded that the acute bronchoconstrictor response to flax dust is increased by pre-harvest retting, suggesting an increased risk of byssinosis.