Serum galactosyltransferase isoenzyme II (GT-II) was assayed in 409 coded serum samples obtained from the National Cancer Institute Tumor Serum Bank using a monoclonal antibody-based immunoassay. The serum panel consisted of samples from patients with confirmed, metastatic ovarian, breast, stomach, esophageal, pancreatic, lung, colorectal, bladder, prostate, and cervical cancer, as well as benign disease controls corresponding to each cancer type, and confirmed healthy normal controls. The serum panel was matched for age and sex; 176 of 179 cancer patients had metastatic disease, and many had undergone previous therapy. GT-II was significantly elevated (P less than 0.01) in all pairwise tests (Wilcoxon) comparing cancer cases with normals and cancer cases with benign disease cases of the same site. A cutpoint of 200 milliunits of GT-II activity/ml of serum was selected, and only one of 50 normal control sera was elevated above this value, yielding a specificity of 98%. The overall sensitivity of the GT-II assay was 55.3%, with higher sensitivity shown by pancreatic (77%), prostate (65%), esophageal (64%), cervical (59%), and bladder cancer (58%).