The total lipid composition of plasma membranes (PM), isolated by the phase partitioning method from two different oat (Avena sativa L.) tissues, the root and coleoptfle, was compared. In general, the PM lipid composition was not conserved between these two organs of the oat seedling. Oat roots contained 50 mole percent phospholipid, 25 mole percent glycolipid, and 25 mole percent free sterol, whereas comparable amounts in the coleoptile were 42, 39, and 19 mole percent, respectively. Individual lipid components within each lipid class also showed large variations between the two tissues. Maximum specific ATPase activity in the root PM was more than double the activity in the coleoptile. Treatment of coleoptile with auxin for 1 hour resulted in no detectable changes in PM lipids or extractable ATPase activity. Differences in the PM lipid composition between the two tissues that may define the limits of ATPase activity are discussed.The plant PM2 is a critical component ofthe cell, separating the cytoplasm from the apoplasm. It contains proteins involved in the selective transfer of ions and molecules across this membrane. One important protein is the H+-ATPase, whose transport of protons to the apoplast is important both for the subsequent uptake of sugars and amino acids by proton-organic cotransport and as a component of growth regulation (28). The activity of this enzyme, as well as other PM proteins, is influenced by the composition of the lipid phase in which they are embedded. This influence is indicated both by experiments where proteins are reconstituted into synthetic bilayers of differing lipid composition (8,17,29) and by experiments where lipid components are partially removed by surfactants (24).