The effect of ex vivo ischaemia at 37 degrees C on the activities of human hepatic acyl-CoA:cholesterol acyltransferase (ACAT), acyl-CoA hydrolase and carboxylesterase and on the microsomal cholesterol and total phospholipid concentrations was determined in liver tissue from two patients. ACAT activity decreased exponentially with ischaemia, corresponding to half-lives of 59 and 54 min in the two samples. Acyl-CoA hydrolase activity of microsomes was not affected by ischaemia, whereas carboxylesterase activity of microsomes decreased with a corresponding increase in cytosolic carboxylesterase activity. No changes in microsomal cholesterol and total phospholipid concentrations corresponding to the changes in ACAT or carboxylesterase activity were observed. ACAT activity was also determined in microsomes prepared from twenty human liver samples obtained at surgery with in vivo warm ischaemia times ranging from 5-120 min. The data obtained agree well with the ex vivo results, showing a half-life of 57 min for the loss of ACAT activity. Therefore, in comparing ACAT activities in liver samples with different ischaemia times, an appropriate correction must be made for warm ischaemia time.