Human milk contains a bile salt-stimulated lipase in amounts that, at pH 6.5 and in the presence of bile salts, might account for a total hydrolysis of the milk triacylglycerols in less than 30 min. In the absence of bile salts the enzyme has no activity against milk fat or against emulsified trioleylglycerol. The primary bile salts sodium cholate and sodium chenodeoxycholate and their taurine and glycine conjugates, but not the secondary bile salt sodium deoxycholate or its taurine and glycine conjugates, caused a pronounced activation of the enzyme against emulsified trioleylglycerol. The lipase was stable at pH 3.5 and 37 degrees C for 1 hour. It was inactivated when incubated with trypsin or chymotrypsin at pH 6.5 but these inactivations were almost abolished in the presence of bile salts. High concentrations of pepsin slowly inactivated the enzyme at pH 4.0. The bile salt-stimulated lipase in human milk is thus stable enough to be active in the intestine, and it is present in high enough activity to contribute significantly to the hydrolysis of the milk triaclyglycerols in the intestine.