Lipase from Pseudomonas stutzeri PL-836 was immobilized on hydrophobic supports and evaluated in the transesterification of wood sterols in solvent-free and solvent-containing media. Triton X-100 was used as additive during immobilization in butyl and octadecyl sepabeads increasing enzyme activity yield by 5% and 60%, respectively. Hyperactivation was observed during immobilization in EC octadecyl sepabeads with enzyme activity yield of 200% and protein immobilization yield of 93%. Thermostability of the immobilized enzyme was assessed at 50 A degrees C in different media in the absence and presence of exogenous solvents. The presence of Triton X-100 during immobilization reduced enzyme stability while tert-butanol increased it. Transesterification in solvent-free and solvent-containing medium with lipase immobilized in EC octadecyl sepabeads showed that the presence of exogenous solvent increased both conversion yield and productivity. At rather high levels of biocatalyst hydration (40% on wet basis) the presence of tert-butanol in the reaction medium more than doubled conversion yield and productivity.