Two laboratory workers, who spent a considerable time each day handling grain infested by the grain weevil ( Sitophilus granarius ), developed allergic responses to this insect varying from rhinitis and pruritus to marked asthma. These findings suggested that weevil protein present in mill dust could result in sensitization in those exposed continuously. A pilot study was therefore undertaken on 75 volunteer millworkers to determine whether such sensitivity existed. A millworker was defined as anyone who worked in a flour mill or mill producing animal feed from mixed cereals, or who worked in grain-storing silos. Skin testing with weevil, mixed flour extracts, and a control was carried out on all 75 volunteers; 57% had a positive response to the weevil extract and 68% a positive response to the mixed flour extract. In a control group of 100 workers from two engineering firms matched for age and sex, 34% were positive to the weevil extract and 17% to mixed flour. From the initial 75 millworkers, 15 were selected for further study based on a positive skin response to the weevil and a history of a productive cough and chest tightness and wheezing when exposed to mill dust. The forced expiratory volume in one second (F.E.V. 1·0 ) was measured after control inhalations and after weevil and mixed flour inhalations. Significant reductions of 20% and 15·4% were found in two subjects after inhalation of weevil extract. In one case wheezing and cough developed. The changes in F.E.V. 1·0 were reversed after inhalation of a bronchodilator aerosol. Twenty-five of the control subjects with positive skin responses to the grain weevil were given similar provocation inhalations but none showed any significant change in F.E.V. 1·0 . This pilot study suggests that grain weevil sensitivity is an additional factor in millworkers9 asthma.