Aerosols were studied in situ and capture on membrane filters from an aircrfat flown around Mount St. Helens during its phreatic period in April 1980. Bulk samples of volcanic ash were collected at ground level 120 km downwind on May 19, 1980 and reaerosolized in a laboratory in ash cloud simulation studies. The aerosol and/or ash samples were tested for ice nucleus (IN) activity using four different IN measurement systems (NCAR acoustical counter, bulk drop freezing, NCAR dynamic thermal diffusion chamber, and filter drop freezing). Although threshold IN activity was observed at -8¿C in bulk ash, in aerosols there were few IN active at temperatures warmer than -12¿C. At -12¿C, IN concentrations were less than 0.4 l-1 (400 m-3) even when the aerosol concentrations were as high as 3000 μg m-3. At aerosol concentrations of 500 μg m-3 and less, the IN content of the ash was below background threshold temperatures of -18¿C.