Purified carboxyl ester hydrolase (carboxylic-ester hydrolase, EC 3.1.1.1) from human pancreatic juice was found to hydrolyze triacetin, methyl butyrate and glycerides solubilized by bile salts. It has no activity on substrate presented as emulsoin or monomolecular films. The human enzyme was found to deacylate phospholipids and lysophospholipids at different rates. The hydrolysis of short-chain phosphatidylcholines was dependent of substrate solubility and dioctanoyl phosphatidylcholine was deacylated with the highest rate. Long-chain phosphatidylcholines and lysophosphatidylcholines present in microsomal membranes were deacylated with very low rates, only lysophosphatidylcholine deacylation was faster. Evidence is presented that human carboxyl ester hydrolase is the lyophosphatidyl-choline-hydrolyzing enzyme corresponding to bovine lysophospholipase. Bile salts play an important part on the activity of human carboxyl ester hydrolase, in addition to the role of detergent that they have on insoluble substrates.