This article reviews the clinical usefulness of three monoclonal antibody-defined tumor markers: CA 19-9 or GICA, CA 50, and CA 125. These markers have been regarded as worthwhile tools for diagnosis and monitoring the management of patients with cancers in selected sites. The CA 19-9 test in combination with the CEA test is a useful adjunct for staging in some advanced cases and for monitoring therapy in the majority of patients with carcinoma of the stomach. Sensitivity of these assays performed concurrently is comparable to CEA alone in colorectal carcinoma. The CA 19-9 test alone is useful for staging and monitoring management of patients with carcinoma of the pancreas. In colorectal carcinoma the CA 19-9 test is redundant because of significantly lower sensitivity than that of the CEA assay; the latter remains the test of choice. The CA 50 test per se is redundant since the CA 19-9 antigen is the target for both the C50 MAb and the NS 19-9 MAb. The CA 125 test contributes to staging and is a useful adjunct for monitoring management of patients with non-mucinous carcinomas of the ovary. If positive after initial surgery and chemotherapy, this test provides evidence of the presence of residual or metastatic tumor and thus may obviate the need for second-look surgery. These conclusions are based on a review of recent relevant publications as well as on our own results obtained from preoperative evaluation and postoperative follow-up of about 600 patients with cancers in relevant sites.