An immune serum generated in Swiss mice against an aqueous preparation from neem leaf was reactive with carcinoembryonic antigen (CEA) and a peptide sequence derived from it. Using ELISA, we have demonstrated that CEA reactive antibody titer (chiefly IgG2a) was significantly decreased after absorption of the immune sera with CEA. Neem leaf preparation (NLP) generated immune sera was also reactive with CEA in immunoblotting and CEA reactive component in the NLP sera can be immunoprecipitated. Identical recognition of CEA expressed on human colorectal cancer specimens, by anti-CEA monoclonal antibody and NLP sera was documented by immunohistochemistry. NLP generated sera could also react with NLP in ELISA and this reactivity was decreased after absorption of the NLP with anti-CEA antibody. Estimation of protein in NLP revealed the presence of it, at least 10% of the total dry weight. In addition, existence of flavone and quercetin was also evidenced from LC-MS analysis. Further studies are needed to identify the antigenic component may have some homology with CEA molecule. This unique property of neem may be utilized for the immunotherapy of CEA positive tumors.