Insulin was precipitated from solution in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) using supercritical carbon dioxide (CO2) as an antisolvent. Biosynthetic human insulin crystals were dissolved in HFIP and the solution was sprayed through an ultrasonic nozzle into supercritical CO2. The factors in the 2(3) factorial experimental design with a center point replicate included pressure (83.7 and 97.5 bar), solution concentration (15 and 30 mg/mL), and solution flow rate (2 and 4 mL/min). Temperature (37 degrees C), CO2 mass flow rate (137 g/min), and volume of solution sprayed (20 mL) were held constant. High-performance liquid chromatography, circular dichroism spectroscopy, infrared and Raman spectroscopy, scanning electron microscopy, dry powder size distribution analysis, thermogravimetric analysis, and atomic absorption spectroscopy were used to characterize the processed insulin powder. The processed insulin retained its potency, was slightly degraded chemically, and exhibited reversible structural changes. The precipitated powder consisted of physical aggregates of 50-nm spheres. Through deagglomeration of these aggregates, it may be possible to obtain discrete uniform particles (1-5 microm) suitable for pulmonary therapy. Over the ranges of operating variables studied, the factors chosen for the experimental design had little effect on the product characteristics. Copyright 2002 Wiley-Liss Inc.