Abstract 1. 1. Urease was entrapped in egg lecithin liposomes. About 1 mg of protein was incorporated per 10 mg of lipid, which corresponds to a molar lipid: protein ration of about 4600. 2. 2. The entrapment procedure led to a change of the apparent Michaelis constant from about 68 mM (free enzyme) to about 167 mM (entrapped enzyme). The V (maximal rate) value did not change appreciably when the enzyme was entrapped. 3. 3. The entrapped enzyme is less sensitive to the pH of external medium than the free enzyme. 4. 4. The entrapment procedure stabilizes the enzyme. The activity of the free enzyme was completely lost in eleven days, whereas the entrapped enzyme lost only 50% of the original activity after twenty days at room temperature. 5. 5. Arrhenius plots of free enzyme activity were linear and the energy of activation ( E act ) was about 7.5 kcal/mol. The plots for entrapped enzyme exhibit a break at about 30°C. The E act values were 8.9 and 17.5 kcal/mol above and below 30°C, respectively. 6. 6. The entrapment protects the enzyme against heat inactivation. The half-times for the decay of specific activity after preincubation at 70°C, were 121 min and 170 min for the free and entrapped enzyme, respectively.