Human monocyte-specific esterase (MSE) derived from leukaemic AMoL-M5 blast cells was purified to homogeneity by the sequential application of anion-exchange, hydrophobic interaction, affinity and gel filtration chromatographic procedures. The resulting enzymatically active MSE primarily existed as an apparent trimer which, under both reducing and non-reducing conditions, dissociated to an inactive 63.4 kD glycoprotein monomer. Electrophoretic studies further confirmed that purified MSE comprised a narrow series of pI (5.5-6.1) forms and one main charge species. Neuraminidase failed to modify observed pI values for individual MSE isoenzymes, and endoglycosidase H treatment revealed that the deglycosylated form of MSE had an apparent molecular weight of 60.1 kD. In support of the known cytochemical characteristics of human MSE, substrate kinetic studies demonstrated that purified enzyme hydrolysed esters of higher acyl chain length (butyrate > propionate > acetate) but did not show peptidase activity. Amino acid sequencing of the MSE N-terminus further revealed that there was almost complete identity with human alveolar macrophage esterase and close similarities with rat and rabbit liver carboxylesterases. These kinetic and molecular studies are particularly important in elucidating the biological and functional role(s) of one of the few haemopoietic cell enzymes that can be considered truly lineage-specific.