TAG-72 is a tumor-associated antigen identified by the monoclonal antibody B72.3. Serum levels of TAG-72 were measured in patients with non-malignant and malignant disease. TAG-72 is not a specific marker of cancer and slightly elevated levels of this antigen can also be detected in the serum of healthy subjects. However, our results show that specificity (92%) and positive predictive value (86%) of this marker are very high. TAG-72 levels above the cut-off limit of 6 U/mL were found in patients with tumors of various organs, including gastrointestinal, ovarian, lung and breast cancer. TAG-72 assay sensitivity is related to tumor stage with values being highest with advanced disease, especially in patients with gastric cancer and lung adenocarcinoma.