We have measured esterase activity levels in microsome products using fluorescein diacetate as a substrate. A comparison of control microsome preparations to human liver microsomes (HLM) shows that the activity levels are much lower (~100x) for the cDNA-expressed enzyme microsomes relative to HLM. Pooled HLM (452161 (Old H161)) 48 nmol/mg/min. SUPERSOMES™ (456201 (Old P201)) 0.7 nmol/mg/min. Lymphoblast (455101 (Old M101A)) 0.2 nmol/mg/min.