Monoglyceride lipase catalyzes the last step in the hydrolysis of stored triglycerides in the adipocyte and presumably also complements the action of lipoprotein lipase in degrading triglycerides from chylomicrons and very low density lipoproteins. Monoglyceride lipase was cloned from a mouse adipocyte cDNA library. The predicted amino acid sequence consisted of 302 amino acids, corresponding to a molecular weight of 33,218. The sequence showed no extensive homology to other known mammalian proteins, but a number of microbial proteins, including two bacterial lysophospholipases and a family of haloperoxidases, were found to be distantly related to this enzyme. By means of multiple sequence alignment and secondary structure prediction, the structural elements in monoglyceride lipase, as well as the putative catalytic triad, were identified. The residues of the proposed triad, Ser-122, in a GXSXG motif, Asp-239, and His-269, were confirmed by site-directed mutagenesis experiments. Northern blot analysis revealed that monoglyceride lipase is ubiquitously expressed among tissues, with a transcript size of about 4 kilobases.