Lipase was immobilized with adsorption or covalent binding on silica particles, being comparatively assayed for esterification in an organic medium. The results show that adsorption, when compared with covalent binding, immobilizes lipase as larger pores are left on the particles, i.e, the extent of plugging of larger pores is less, This dependency suggests that the former proceeds within only larger pores on the particles, while the latter proceeds within all pores. Correspondingly, the former immobilizes lipase in quantities half that of the latter. Nevertheless the former is able to produce more active preparations than the latter. Both kinds of preparations vary their activities with an increase in their water contents with similar convex functions, having respective maximum activities at water contents equivalent to their pore volumes.