The marine flagellate Dunaliella marina responds to hypotonic media in the same way as animal cells: an initial phase of osmotic swelling occurs, followed by a second, more prolonged phase of cell shrinkage - the volume-regulation phase. D. marina also volume-regulates in hypertonic media. This does not usually occur in animal cells exposed to normal concentrations of K+. Copper concentrations of 2 ppm and higher inhibit regulation of cell volume in D. marina transferred to hypotonic media. This is probably due to reduced cell-membrane permeability to K+ and/or Cl- and not to an interference with the Na+/K+ exchange pump. These findings suggest that volume regulation is functionally independent of the Na+/K+ exchange pump. The method described herein appears to be a suitable biotest for pollutants, providing specific information on their effects on the permeability of cell membrane.