rhDNase is a recombinant human protein approved as an aqueous solution for human use by inhalation. To study the feasibility of preparation of dry powders for inhalation, spray-dried powders of pure rhDNase and co-spray-dried mixtures of rhDNase with an excipient approved for inhalation products, lactose, were prepared. Both types of powders were initially amorphous. The lactose, however, was found to crystallize after exposure to a humid environment. The crystals in the powder were identified as the alpha-monohydrate polymorph of lactose by hot-stage optical and scanning electron microscopy, differential scanning calorimetry and thermogravimetry, FTIR spectroscopy, and X-ray powder diffraction. Moisture sorption isotherms indicated that crystallization occurred at high relative humidities (70-85%), depending on the temperature of the environment (5-40 degreesC). The practical implications for the manufacturing and storage of protein powders for inhalation are discussed.