L-histidine ammonia-lyase (histidase) was encapsulated within cellulose nitrate artificial cells, and its kinetic parameters were evaluated. Microencapsulated histidase had an apparent activity of histidase in solution. Encapsulation did not alter the Km of histidase. The Km of histidase solution and the Km apparent of micro-encapsulated histidase were both 20 mM. Encapsulation of histidase resulted in increased stability of enzymatic activity at storage temperatures of 4 degrees C and 37 degrees C. At 37 degrees C histidase solution reached 50% of its original activity after 9.5 days of storage, while microencapsulated histidase reached the same level after 15 days. At 4 degrees C histidase solution had 63% of its original activity after 21 days of storage, while encapsulated histidase had 95%. In vitro experiments to evaluate the feasibility of microencapsulated histidase for possible experimental therapy in histidinemia were carried out. These experiments evaluated the effectiveness of encapsulated histidase in depleting histidine. Three different volume ratios of histidase loaded artificial cells to substrate solution were tested. A ratio of 1: 100 allowed 25% histidine depletion after 120 hours. A 1: 50 ratio allowed 35% histidine depletion after 72 hours. A 1: 25 ratio allowed 40% histidine depletion after 24 hours (Int J Artif Organs 1990; 13: 189-95).