In a two-phase jet, the rise height of particles is strongly dependent on initial particle concentration and particle size because the magnitude of the drag force varies with these two factors. Large particles (those with terminal fall velocities comparable to initial jet velocity) and small particles (terminal fall velocities much less than jet velocity) react to the drag force in different ways. As their concentration is varied, small particles reach a minimum in rise height when their initial particle-field density equals the fluid density. The rise height of large particles varies little with concentration. The results may be applicable to explosive volcanic eruptions because the lower part of an eruption column is jetlike rather than plumelike. One implication is that eruption column collapse and pyroclastic flow formation, caused by gradual changes in initial particle concentration or gas content, may be modified by drag effects, perhaps accounting for column height fluctuations during collapse and some of the bedding characteristics of coignimbrite lag breccias. ¿ American Geophysical Union 1989