Esterases are enzymes capable of hydrolyzing the simpler esters of N-free alcohols and organic acids. The majority of esterase enzymes are able to hydrolyze a synthetic ester substrate that is -naphthyl acetate; so they are called non-specific esterases. Nonspecific esterase activity was visualized from various types of cancer cells, where their activity showed to decrease in association with different tumor types. Aim; The aims of this study were to establish the normal esteratic profile in blood biochemically and in monocytes and lymphocytes cytochemically, study the esteratic pattern and the isoenzymatic state of esterases in patients with cancer, and characterization of esterase-substrate reaction using kinetic and thermodynamic parameters. Materials; All chemicals and standard solutions used in this work of the highest analytical grade, obtained from commercial sources and used without further purification. Hexazotized pararosaniline dye, phosphate buffer, -naphthyl acetate, polyacrylamide gel, and Biuret kit were used. Patients and Methods; Ninety seven patients with proven malignancy were investigated from Al- Kadhimiya Teaching Hospital / Department of Oncology, in Baghdad during the period from January 2004 till October 2005. Thirty six patients were diagnosed with adenocarcinoma of the breast, twenty nine patients with adenocarcinoma of III the bronchus, twenty one patients with lymphoma - including Hodgkin's and non-Hodgkin's lymphoma -, and eleven patients with adenocarcinoma of the colon), and compared with twenty normal male and female individuals. The samples were studied at the Departments of Physiological Chemistry and Human Anatomy, College of Medicine, Al-Nahrain University. The isoelectric focusing electrophoresis was used for studying the esteratic pattern in normal and cancer patients using slab gel and cylindrical gel techniques. The images generated by electrophoresis were processed using PhotoCaptMwt_ Software, and used in determination of enzyme units in standard carboxylesterase (E.C. 3.1.1.1) and all studied individuals. Membrane-associated human monocyte carboxylesterase was extracted from peripheral blood sample of breast cancer patient, by using monocyte isolation by adherence method. Spectrophotometric method was used to study the reaction of esterase enzyme with -naphthyl acetate substrate, and Michaelis-Menten constant (KM) and maximum velocity (Vmax) were determined. The thermodynamic parameters (DH, DS, and DG) of the reaction were determined. Blood smears were taken from both control and patients groups, and stained with azo-coupling dye (hexazotized pararosaniline) using -naphthyl acetate esterase as a substrate. Monocyte nonspecific esterase activity was quantitatively measured using microspectrophotometry technique. Results; The isoelectric point of the plasma fraction for the patient groups showed higher values (pH 7.2 _ 7.5), while it was 6.3 in control group (male and female). Lysate fraction for all patient groups showed band of pH ranges from 5.3 _ 6.3, 8.2 _ 9, and 9 _ 9.8.