Unsaturated fatty acyl chains of Dunaliella salina membrane lipids can be catalytically reduced by the homogeneous hydrogenation catalyst palladium di(sodium alizarine monosulphonate), Pd(QS)2, under conditions permitting full recovery of the cells within 24 h. The hydrogenation is accomplished by incubation of cells with the hydride form of Pd(QS)2 under 1 atmosphere of H2 and for 2 min or less. Following this protocol, hydrogenation reduces only those fatty acids located in the plasma membrane and other membranes located near the cell surface. The limited reactivity in vivo is due to the fact the Pd(QS)2 permeates into the living cells more slowly than it does into liposomes prepared from extracted Dunaliella membrane lipids. While Dunaliella is completely unaffected by exposure to the oxygenated, inactive catalyst, hydrogenated cells cease growth for approximately 12 h, during which time the hydrogenated acyl chains are being enzymatically retroconverted to their original unsaturated form. When the lipid composition approaches its prehydrogenation values, growth resumes, presumably due to the restoration of normal membrane functions. The system shows promise for studying the metabolic regulation of membrane microviscosity.