Particles in the size range of 0.1--25 micrometers were sampled by aircraft carrying a quartz crystal microcascade in the Mount St. Helens plume on three dates in August and September 1980. Two of the sampling dates represented 'typical' emissions of the volcano between plinian eruptions. One sampling flight was made 1--4 hours before the small plinian eruption of August 7, 1980, when the plume had become discontinuous and visibly darker. Size distributions were determined, and individual particles were studied by using scanning electron microscopy. The plume sampled on August 7, before the eruption, contained mainly approximately 2 micrometer diameter silicic glass particles, fragments of the Mount St. Helens magma. The 'typical' plumes sampled on September 22 and August 6 had much smaller concentrations of particles, trimodal size distributions with peaks at 10, 0.4, and 0.1 micrometers. The particles were largely nonsilicates and apparently represented Cu--Zn oxide (10 micrometer peak), Al sulfate, chloride and oxide (?), and sulfuric acid (smallest size peak). The characterization of small particles in the plume may give information about approaching activity and possibly about changing conditions in the subsurface magma body.