Aspartate transcarbamylase (carbamoyl phosphate: Laspartate carbamoyltransferase, E.C. 2.1.3.2.) activity from a thermophilic strain 7-11-05 of Chlorella pyrenoidosa appears to be stabilized during and after (NH₄)₂SO₄ precipitation of the enzyme by at least two factors, one having a molecular weight greater than 10,000 and the other having a molecular weight less than 10,000. The high molecular weight factor(s) appears to be an albumin-like protein which is important in stabilizing the activity of the reconstituted enzyme during storage in buffer. The low molecular weight factor(s) is necessary for stabilization of enzyme activity both during and after (NH₄)₂SO₄ precipitation, and it appears to be organic in nature. Stabilization of enzyme activity in whole son1cates during aging appears to be dependent on a heat-stable factor(s) which has a molecular weight of less than 1,000. Aspartate transcarbamylase activity can be stabilized and modulated in vitro by a number of different compounds such as carbamoyl phosphate, UMP, uridine, and cytidine. This low molecular weight stabilizing factor(s) is suggested to be similar to the low molecular weight factor(s) necessary for stabilization of the enzyme during and after (NH₄)₂SO₄ precipitation. The decay of enzyme activity during aging in vitro is proposed to be due to a breakdown of a polymeric form of aspartate transcarbamylase into subunits which have greater catalytic activity than the native enzyme and which are labile 1n the absence of a stabilizing factor(s). An active breakdown of a stabilizing factor(s) could not be observed in whole sonicates, and proof of an active breakdown of the enzyme itself must await further experimentation. / Master of Science