Sera from rats bearing primary or grafted colon carcinoma may contain antibodies that can react with antigenic determinants at the surface of cultivated colon cancer cells. Assays with various target cells and absorption experiments suggest that antigens recognized by circulating antibodies are common to independent lines of cultivated colon cancer cells. They are cross reacting, tumor type specific antigens. They could be embryonic or fetal antigens, because some sera from multiparous animals react with colon cancer cells. Blocking experiments suggest that these antigens differ from the carcinofetal antigen previously demonstrated on the surface of intestinal cancer cells by xenoantiserum.