The effects of carrier particle size, carrier ratio and addition of fine ternary agents on dry powder inhalation performance of insulin-loaded PLGA microcapsules were investigated. The use of mannitol with larger particle size in the lower carrier/microcapsules ratio led to higher deposition profiles of the drug. The fine spray dried mannitol and spray dried leucine as the ternary agents both decreased the deposition and dispersibility of microcapsules. It was concluded that addition of ternary component does not always improve aerosolization properties of DPI formulations. Carrier particle size, its flowability and carrier-drug ratio were shown to play an important role in DPI formulations. (C) 2010 Elsevier B.V. All rights reserved.