The humoral immune reactivity was studied in patients affected by gastro-intestinal cancer. The number of peripheral B lymphocytes, the concentration of serum immunoglobulins (Ig) and of C3 complement factor and the frequency of circulating immuno complexes (CIC) did not significantly differ between patients and age-matched controls, while the C4 factor level was significantly increased. The frequency of serum monoclonal components (M-components) was higher in the patients than in the elderly subjects. Moreover the results concerning the "in vitro" functional response of patient B lymphocytes showed a significant decrease of the proliferative responses to Staphylococcus Aureus Cowan I (SAC) and a significant increase of the IgG and IgM synthesis by peripheral blood mononuclear cells (PBMC) in unstimulated and pokeweed mitogen (PWM) stimulated cultures. The meaning of these results, taken together with those reported by others, is discussed.