The circulating Carcinoembryonic antigen (CEA) level apparently depends upon several factors, including the pathological stage and degree of differentiation of the primary tumor, the presence of invasion of lymphatics, blood vessels or perineural spaces, the extent of distant spread of tumor. Alphafetoprotein (AFP) is a glycoprotein of between 65,000 and 70,000 daltons first identified by Bergstrand Czar in 1956 as X-component in human cord blood. Both CEA and AFP are sufficiently good antigens when injected into animals such as goats, rabbits, guinea pigs, sheep, etc., so that after appropriate absorption, antisera of high titer and affinity can be obtained. The same situation appears to prevail in immunizing mice for subsequent somatic cell fusion. Despite the similarities in immunization and fusion protocols used by various investigators, some differences in the antibodies obtained have become apparent. The monoclonal antibody was found useful in immunohistochemical studies for the localization of CEA in tissue sections and colonic cancer cells.