Mixtures of microbubbles (particle size, 20 mu m) with water or dilute polymer solutions were passed through micro-orifices of 100-400 mu m. Pressure drops were measured, and elastic stresses were estimated. The pressure drops of Polyethylene Oxide solutions alone were less than those of water, while the pressure drops of microbubble mixtures with Polyethylene Oxide were greater than those of Polyethylene Oxide solutions alone. This phenomenon was thought to arise from hindrance by microbubbles of the association of polymer chains in the solution; and this idea was supported by the estimated elastic stresses.