Two tomato (Lycopersicon esculentum Mill. cv. Pera) callus lines tolerant to NaCl were obtained by successive subcultures of NaCl-sensitive calli in 50 and 100 mM NaCl-supplemented medium. Growth and ion content, as well as plasma membrane lipid composition, fluidity and H+-ATPase (EC 3.6.1.35) activity, were studied in both NaCl-sensitive and NaCl-tolerant calli. Although calli tolerant to 100 mM NaCl exhibited a reduced growth relative to calli sensitive to NaCl or tolerant to 50 mM NaCl, growth of calli tolerant to 100 mM NaCl was higher than that of NaCl-sensitive calli grown for one subculture in 100 mM NaCl. Growth in the presence of 100 mM NaCl provoked an increase of Na+ and Cl- content, but no significant changes in K+ and Ca2+. As compared with NaCl-sensitive and 50 mM NaCl-tolerant calli, plasma membrane vesicles isolated from calli tolerant to 100 mM NaCl exhibited a higher phospholipid and sterol content as well as a lower phospholipid/free sterol ratio and a lower double bond index (DBI) of phospholipid fatty acids. The changes in plasma membrane lipid composition were correlated with a decrease of plasma membrane fluidity in calli tolerant to 100 mM NaCl, as indicated by fluorimetric studies using diphenylhexatriene (DPH) as probe. Plasma membrane-enriched vesicles isolated from calli tolerant to 100 mM NaCl showed lower ATP hydrolysis and ATP-dependent H+-pumping activities, as well as a lower passive permeability to H+ than plasma membrane from NaCl-sensitive and 50 mM NaCl-tolerant calli. The involvement of the changes in plasma membrane lipid content and composition, fluidity and H+-ATPase activity in salt tolerance of tomato calli is discussed.