Recombinant human interferons have recently been shown to enhance tumor antigen expression, including carcinoembryonic antigen (CEA), on the surface of human carcinoma cells, which results in an increase in the targeting of antitumor monoclonal antibodies (MAb) in vivo. We report here the effect of recombinant human gamma-interferon (HuIFN-gamma) on the expression of human CEA and its related transcripts in several human colon carcinoma and normal human fibroblast cell lines. The colon tumor cell lines HT-29, WiDr, and LS-174T were each shown to express different constitutive levels of CEA glycopeptide, as measured by the binding of the CEA-specific MAb COL-4. Treatment with HuIFN-gamma enhanced the level of binding of COL-4 in total cell extracts of HT-29 and WiDr cells 2.5- and 6.5-fold, respectively. Using a CEA complementary DNA probe, this increase in MAb binding was shown to be accompanied by a 6- to 11-fold increase in the steady state levels of three CEA transcripts with sizes of 4.2, 3.5, and 2.8 kilobases. On the other hand, HuIFN-gamma treatment had no effect on the level of COL-4 binding or expression of CEA transcripts in LS-174T colon carcinoma cells, which are high constitutive expressors of CEA glycoprotein. Normal human fibroblast cell lines MRC-5 and WI38 had no detectable cytoplasmic CEA glycopeptide levels nor did they contain detectable levels of CEA mRNA, either before or after treatment with HuIFN-gamma. In contrast, HuIFN-gamma induced the de novo expression of the normal major histocompatibility complex class II antigen, HLA-DR, on HT-29 and WiDr colon cancer cells as well as the two fibroblast cell lines. Treatment of the LS-174T cell line with HuIFN-gamma did not result in the induction of class II HLA-DR antigen. These observations suggest that some common factors may be involved in the regulation of the CEA and class II histocompatibility genes. In addition, the demonstration that HuIFN-gamma enhances CEA expression in some carcinoma cell lines but fails to induce de novo expression of CEA transcripts in fibroblasts supports the potential application of HuIFN-gamma in enhancement of tumor targeting of antitumor MAbs and adds to our understanding of the mechanism of gamma-interferon-mediated up-regulation of some tumor antigens.