Monoclonal antibodies wee obtained by the immunization of mice with 6'LM1 (IV6NeuAc-nLcOse4Cer) adsorbed to Salmonella minnesota. The monoclonal antibodies showed a specificity for gangliosides with a terminal NeuAc alpha 2-6Gal substitution, which was demonstrated in solid-phase binding assay and in liposome inhibition assay. Gangliosides with a NeuAc alpha 2-6Gal substitution were minor components of different normal tissues. However, these gangliosides were enriched in carcinomas of many tissues, and were particularly enriched in most colorectal carcinomas and in lung carcinomas. 6'LM1 is a characteristic ganglioside in fetal intestinal mucosa (meconium). This ganglioside and other gangliosides with a terminal NeuAc alpha 2-6Gal substitution might represent oncofetal antigens expressed in carcinomas owing to an activation of a "fetal' sialyltransferase.