In our previous work, the organic-solvent-soluble enzyme (lipase B) was prepared in high yield using a synthetic detergent. Using this method, lipases from various origins have been solubilized in organic solvents with several kinds of detergent and the influence of detergent attached to the enzyme molecule on the reactivity of the organic-solvent-soluble lipase was investigated. The activities of the organic-solvent-soluble lipases have been compared in a homogeneous solvent system, tetrahydrofuran containing 20% buffer. It is shown that the hydrolysis rate of the organic-solvent-soluble lipase is affected by the nature of the surrounding detergent and that the organic-solvent-soluble lipase prepared from a detergent with a large hydrophobic region hydrolyses hydrophobic substrates more rapidly than that prepared from a detergent with a small hydrophobic region.