AbstractThe human colon carcinoma cell line HT‐29 was adapted to grow in chemically defined medium (CDM). The spent CDM (S‐CDM) was concentrated by Amicon filtration and the crude HT‐29 S‐CDM purified by 40% saturated(NH4)2SO4precipitation. The purified antigen was tested by a micro‐complement fixation (MCF) assay against the sera of cancer patients of various histologic types and against the sera of normal donors. Fifteen of 20 (75%) colon cancer, 16/20 (80%) breast cancer sera, 14/19 (74%) lung cancer sera, and 13/20 (65%) miscellaneous carcinoma sera were positive in the MCF. By contrast, 2/21 (10%) melanoma sera, 7/20 (35%) sarcoma sera, and 2/19 (11 %) normal sera were positive. These data suggest the presence of a carcinoma‐associated antigen in the spent CDM of the HT‐29 colon carcinoma cell line adapted t