The era of searching tumor metabolism in part include the measurement of many markers that could participate in management and understanding the etiopathogenesis of these tumors. CA19-9 is regarded as one of the tumor markers that indicate it's relation to the carcinoma of pancreas, stomach, colon and rectum. The degree of relation differs widely among studies allover the world. aim; Aim of this study is to evaluate the level of CA19-9 in sera and tissue of patients with malignant colorectal lesion, patient with benign tumors and normal control group, and to found the most suitable kinetic and thermodynamic parameters for antigen-antibody reaction, the study included immunoglobulin relations the level of CA19-9in sera of three groups. Materials; All chemicals and standard solutions used in this work of the highest analytical grade, obtained from commercial sources, and used without further purification. Tris buffer, different halide and divalint cations, comassie blue, immunoradiometric assay kit and radial immunodiffution plates. Patients and method; Thirty one patients with colorectal carcinoma, thirty one patients with benign lesions and thirty one healthy subjects (normal control) age matched collected from Hepatology and astroenterology Baghdad Teaching Hospital, and Alsader Teaching Hospital in Al-Najaf city during period from May 2004-April 2005. The samples were studied in the Department of Physiological Chemistry, Colleg of Medicine,Al-Nahrain University. The immuoradiometric assay had been modified in order to be used in studying the binding of CA19-9 with 125I-labeled anti CA19-9 monoclonal antibody in the normal and malignant tissue homogenates. The binding of 125I-labeled anti CA19-9 monoclonal antibody with CA19-9 in serum and tissue of patients with malignant colorectal tumor and normal controls were investigated .Different factors affecting this binding were extensively studied such as. concentration of antigen, concentration of antibody, pH, time, temperature, concentration of salts, and Polyethylene Glycol. The kinetic parameters of the binding antibody-antigen; equilibrium constant of the association ( Ka ), equilibrium constant of the dissociation ( Kd ), association rate constant ( K+1 ), dissociation rate constant (K-1),half life ( t1/2 ), maximum binding( Bmax ), and observed rate constant ( Kobs ) were cured out in the four groups at 4,20,37 and 45CÆ. The thermodynamics of the binding CA19-9 with antibody in the four groups were studied, using Van't Hoff and Arrhenius equations, the thermodynamic parameters of the standard state(?HÆ , ?GÆ , ?SÆ ) and the transition state (Ea, ?HÆ , ?GÆ , ?SÆ ) were determined . The levels of immunoglobulines (IgM, IgA, IgG) in serum of patients with benign and malignant colorectal tumor and normal subjects were measured by radial immunodiffution method. Results; Results obtained indicated that there was a significant increase (P0.001) of CA19-9 in serum and tissue of patients with malignant colorectal tumor as compared with those for patients with benign tumor and healthy subjects. The mean serum level of CA19-9 in patients with the three stages B,C and D were found to be significantly elevated (P 0.01, P 0.001 respectively) and no significant elevation in patients with stage A in comparison with the control group. A significant variation was indicated among patients tissues having malignant tumors of different stages A,B,C and D P 0.01). On the other hand, the mean serum levels of CA19-9 showed significant elevation (Po.o1, P0.001 respectively) in patients with moderately and poorly differentiated groups and no significant elevation was noticed in patients with well differentiated carcinoma in comparison with the control group. However the variation was significant on comparison of data for patients with different grades (P 0.01). Serum and malignant tissue levels of CA19-9 were found to have a positive correlation coefficient (r 0) and occur spontaneously (?GÆ 0) and the binding reaction were entropically driven (?GÆ 0 and ?SÆ 0). Serum IgA levels were found to be significantly elevated in patients with stages B, C and D (P0.01) as compared with patients with benign tumor and normal subjects, no significant elevation were obtained in the levels of serum IgM and IgG of malignant and benign tumor as compared with normal control.