Cells of oat coleoptiles (Avena sativa L. cv. "Garry") have been osmotically shocked in order to observe the effect of alterations of the plasma membrane on some auxin responses. When coleoptile sections were treated sequentially with 0.5 m mannitol and 1 mm Na-phosphate (pH 6.4) at 4 C, polar auxin transport and acidification by 1 mM CaCl(2) were unaffected, but auxin-stimulated acidification and growth were eliminated. Shock treatment also had no effect on acid-stimulated growth or on freezing point depression by the cytoplasm. It is suggested that osmotic shock modifies a portion of the plasma membrane which interacts with auxin and eventually leads to growth.