The esterases of human leukocytes were studied both by cytochemical methods and by polyacrylamide gel electrophoresis. The aim of the study was to clarify the specificity of various cytochemical methods and to establish specific methods for demonstrating the various types of esterases in the blood cells. Nine bands of isoenzymes were separable by polyacrylamide gel electrophoresis at pH 4.0. Bands 1, 2, 7, 8 and 9 were best demonstrated by naphthol AS-D chloroacetate and represented the activity of chloroacetate esterase as demonstrated by cytochemical methods in the granulocytes. Bands 3, 4, 5 and 6 were best demonstrated by naphthyl esters and represented "nonspecific esterases" as demonstrated by cytochemical methods in the monocytes, platelets and plasma cells. "Aminocaproate esterase" or "trypsin-like enzyme" was demonstrated only in the human mast cells. Its exact location in the gel system is unknown since pure human mast cell preparation was not available for study. These three groups of esterases had a substantial but not absolute substrate specificity. They can be specifically demonstrated in the various types of human blood cells only under properly controlled staining conditions.