A simple method for the preparation of S-propranolol catalyzed by a Rhizopus niveus lipase in an aqueous medium is described. Hydroxypropyl-p-cyclodextrin was used for the first time to increase the solubility of (R,S)-O-butyryl propranolol thus permitting the reaction to be carried out in water. The formation of an inclusion complex between (R,S)-O-butyryl propranolol and hydroxypropyl-beta-cyclodextrin was studied and a stoichiometry of 1:1 was determined. The influences of the hydroxypropyl-beta-cyclodextrin concentration, pH and percentage of ethanol on the enzymatic activity were also investigated. Under the conditions presented in this paper, values of ee(s) of 90% and E=48 were obtained.