Normal human leucocytes and platelets contain esterases which hydrolyse alpha-naphthyl acetate (alpha NA). Purified preparations from these cells were investigated by isoelectric focusing and subsequent polyacrylamide gradient gel electrophoresis at pH 9.0. Extractable alpha NA esterases were separated according to isoelectric point (pI) and molecular weight (MW). Monocytes, lymphocytes, granulocytes and platelets contain a unique pattern of alpha NA esterases, most of which can be inhibited by diisopropyl fluorophosphate (DFP; 0.1 mM). Their activity, however, is not affected by eserine (0.1 mM) or p-hydroxymercuribenzoate (1 mM). No protease activity of these enzymes was detected; it is likely that the majority constitute carboxylesterases (EC 3.1.1.1). Monocytes contain five alpha NA esterases which are additionally inhibited by bis(4-nitrophenyl)-phosphate (0.1 mM) and sodium fluoride (40 mM). PIs are in the range 5.7-6.2 and MWs are 145 000, 155 000, 250 000, 290 000 and 340 000. These enzymes are specific for monocytes. Platelets are characterized by a group of alpha NA esterases having pIs between 6.5 and 8.0, these corresponding to MWs ranging from 15 000 to 400 000.