This study was carried out to investigate if particles approximately less than 5 microns are optimal when delivered from a dry powder inhaler. It was performed as a cumulative dose response study of twelve asthmatic adults. Three different versions of a dry powder inhaler, Bricanyl Turbuhaler, were studied. Each inhaler delivered 0.5 mg of pure terbutaline sulphate with each dose. Out of this 0.5 mg, they delivered 90 micrograms, 40 micrograms and 5 micrograms, respectively, of particles approximately less than 5 microns with each dose at an inspiratory flow rate of 28 l.min-1. Terbutaline 0.5 mg, 0.5 mg, 1.0 mg and 2.0 mg was inhaled with a 30 min interval between the doses. Forced expiratory volume in one second (FEV1) and forced vital capacity (FVC) were measured 5 and 20 min after each dose. The bronchodilator response was greater with the inhaler delivering 90 micrograms of small particles with each dose than with the inhaler delivering 5 micrograms, thus confirming the importance of small (approximately less than 5 microns) particles.