A tumor-associated protein from the cellular membranes of a radiation-induced rat small bowel adenocarcinoma was identified, found to be serologically unaltered in the circulatory system, and was observed to be susceptible to acid hydrolysis. The immunochemical reactivity was unchanged by heat, alkali, or neuraminidase digestion. The protein appeared to be a single immunologically active species, but it was structurally composed of a heterogeneous group of proteins.