Total poly(A)-containing RNA extracted from a rectal carcinoma was translated in a rabbit reticulocyte lysate. By addition of either a monoclonal or a polyclonal antibody, both monospecific for carcinoembryonic antigen, one protein with an apparent molecular weight of 85,000 was specifically precipitated, as shown by electrophoresis of the immunoprecipitate on sodium dodecylsulfate polyacrylamide gels. This protein behaves similarly to CEA isolated from liver metastases of a colon tumor in its property of being resistant to cleavage by cyanogen bromide. These findings suggest that this protein represents the CEA precursor protein. When we consider the high carbohydrate content of 60% of CEA, the observed molecular weight of the CEA precursor protein is in agreement with the reported molecular weight of 180,000 for CEA. By sedimentation of poly(A)-containing tumor RNA through a sucrose gradient, and by in vitro translation of each fraction of the gradient, the sedimentation coefficient of CEA-specific mRNA was found to be about 22 S.