The molecular size of pig liver carboxylesterase has been investigated under a variety of conditions of pH and ionic strength. From equilibrium and velocity sedimentation at pH 4.0 and pH 7.5, and from chromatography on Sephadex G-200, we conclude that the monomeric molecular weight is ~65 000 daltons and that the enzyme associates to form trimers. Association equilibrium constants for the monomer–trimer system were estimated to be 0.02 l2 g−2 at pH 4 (concentration-dependent molecular weight data) and 2 × 105 l2 g−2 at pH 7.5 (frontal gel chromatographic results). These studies were aided by comparisons of the properties of the pig liver enzyme with those of chicken liver carboxylesterase, which is shown to exhibit the velocity and equilibrium sedimentation characteristics of a homogeneous protein with molecular weight ~65 000. Studies of pig and chicken liver carboxylesterases in 6 M guanidinium chloride, 0.1 M in β-mercaptoethanol, support the proposition that the monomeric species of these enzymes have molecular weights of ~65 000. On polyacrylamide gel electrophoresis in SDS, there is no evidence for a major species of molecular weight less than ~65 000 for the pig enzyme, but ca. 50% of the chicken esterase is dissociated into two species of molecular weight ~30 000.