A formulation and process development study was performed to formulate recombinant human deoxyribonuclease I as a powder for inhalation. First, excipient compatibility (with bovine DNase as a model substance) was examined with a stability study at stressed conditions (60 and 85°C) while monitoring for occurrence of the Maillard reaction. Next, powders for inhalation were prepared by spray drying and spray freeze drying. We found that spray drying with inulin as stabilizer resulted in the best powder for inhalation. Finally, an ex-vivo test with the spray dried rhDNase I/inulin powder significantly decreased elastic and viscous moduli of sputum from five cystic fibrosis patients.