Monocyte nonspecific esterase has been purified from cultured cells of the acute myeloid leukemia cell line, ML-1. The purified enzyme shows the characteristic properties of the monocyte neutral serine carboxyl esterase, with high sensitivity to organophosphorus inhibitors and sodium fluoride inhibitor. The enzyme is a membrane protein which in the native state exists as a monomer of a mol wt of approximately 68,000 and a trimer of mol wt 205,000. These forms exhibit a complex pattern of dissociation and reassociation based on apparent noncovalent binding of subunits. The delipidated dissociated enzyme runs as a single protein chain of a mol wt of approximately 62,000 on sodium dodecyl sulfate (SDS) gel electrophoresis. The relation of the subunits to monocyte isoenzymes seen on isoelectric focusing (IEF) and polyacrylamide gel electrophoresis at pH 9.5 (pH 9.5 PAGE) of cell extracts is demonstrated. Availability of purified enzyme allows development of monoclonal antibodies and analysis of myeloid differentiation. In addition, the substrate specificity and function of the purified monocyte ectoenzyme are being examined.