ABSTRACT Carcinoembryonic antigen is a glycosylated protein used as a human tumor marker to assess recurrences of gastrointestinal, breast and lung cancers. This protein is a member of the immunoglobulin supergene family and has been shown to function as a homophilic and heterophilic cell adhesion molecule. We have studied the spatial and temporal expression of two cloned mouse CEA gene family members during development using Northern analyses and in situ hybridization. Transcripts detected by the mouse CEA probes are expressed from 10.5 days post coitum (p.c.) to birth in mouse embryos and fetuses and are present from 16.5 days p.c. to adulthood in intestine and colon as evaluated by Northern analyses. The RNA is also present in many other tissues including meninges, cartilage and bone, blood vessel walls, placenta, dermis, muscle layers of the stomach and intestine and bronchioles of developing mouse embryos and fetuses. This expression pattern is similar to that of proteins of the Transforming Growth Factor β gene family. The role of mouse CEA gene • family members is unclear; however, the localizations of these CEA family members in the mouse embryo and fetus suggest an important functional role during active morphogenesis, a complex process in which cell adhesion molecules are significantly involved.