Acyl-coenzyme A (CoA):cholesterol O-acyltransferase (ACAT) is responsible for esterification of cholesterol in the cell. The enzyme has never been purified, but two cDNA sequences coding for this enzyme were recently reported. One of the sequences was identical to human liver carboxylesterase. We have used inhibitors to elucidate the relation between microsomal carboxylesterase, acyl-CoA hydrolase (ACH), and ACAT activities in rat liver. Low concentrations of serine esterase inhibitors strongly inhibited carboxylesterase and acyl-CoA hydrolase activities but stimulated ACAT activity. At higher concentrations, ACAT activity was also inhibited. A sulfhydryl-modifying agent was found to be a potent inhibitor of ACAT without affecting carboxylesterase activity. Similarly, two specific ACAT inhibitors, DL-melinamide and PD 138142-15, inhibited ACAT activity but did not affect carboxylesterase or ACH activities. Our data thus exclude ACAT as a liver microsomal carboxylesterase. The complex inhibition patterns observed with serine esterase inhibitors indicate that carboxylesterases and ACHs may interfere with ACAT activity by competing for the substrate. It is obvious that final identification of ACAT requires demonstration of an active homogenous protein.