The recent cloning of complete cDNAs encoding carcinoembryonic antigen (CEA) and non-specific cross-reacting antigen has revealed the existence of a new gene family belonging to the immunoglobulin gene superfamily. We have reported the isolation of a partial CEA cDNA and of L-cell transfectant cell lines that express human antigens cross-reactive with commercial antibodies directed to native CEA (Kamarck, M., J. Elting, J. Hart, S. Goebel, P. M. M. Rae, J. Nedwin, and T. Barnett. 1987. Proc. Natl. Acad. Sci. USA. 84:5350-5354). In this study, we describe the identification and cloning of 3.9-, 3.7-, 2.2-, and 1.8-kb cDNAs and a 23-kb genomic transcription unit, which code for new members of the CEA gene family. DNA sequence analysis of these cloned DNAs establishes the existence of a set of four alternatively spliced mRNAs which are expressed in several tumor cell lines, in human fetal liver, and in L-cell transfectants. Deduced amino acid sequences of the encoded isoantigens show extensive similarity to CEA and nonspecific cross-reacting antigens, but in addition demonstrate transmembrane and cytoplasmic domains. We designate members of this antigen family transmembrane CEAs. The transmembrane CEA isoantigens share general structural characteristics with members of the immunoglobulin gene superfamily and can be specifically compared to the cell adhesion molecules, N-CAM (neural cell adhesion molecule) and MAG (myelin-associated glycoprotein).