Drug delivery to the lungs is an important approach to the treatment of lung diseases. Interactive powder mixtures containing the drug in micronised form adhered to coarse grade carrier particles provide one of the methods. During inhalation the drug particles must be re-suspended in the respiratory air stream to be able to pass through the lower airways, whereas the coarse carrier particles are usually swallowed. Adhesion forces must be overcome during this re-suspension step. If particles are not re-suspended, they will stick to the carrier particles and will be swallowed. However, from re-suspension models it is known that there is a complex relationship between the degree of re- suspension, force of adhesion and particulate properties. Experiments were carried out to measure the adhesion force between drug (Salmeterol Xinafoate) and carrier particles (lactose monohydrate) in interactive powder mixtures. The properties of the carrier particles and the mixing procedure were varied. Powder re-suspension of the mixtures was assessed by determination of the amount of particles detached from the carrier particles and the aerodynamic particle size distribution of the re-suspended drug at an air flow rate of 60 1/min. An increase in median adhesion force as measured by a centrifuge technique resulted in a reduced total amount of re-suspended drug particles, when measured by a cascade impactor, but size and surface roughness of the carrier particles also influenced the aerodynamic properties. A mathematical re-suspension model was developed, which was found to be able to predict the aerodynamic particle size distribution and the total amount of drug re-suspended from the adhesion force distributions measured, the size of the carrier particles, and a measured value for the coefficient of static friction between drug and carrier particles.