A murine monoclonal antibody designated 1G12 has been produced by immunizing mice with A549 human lung adenocarcinoma. Using radiolabeled monoclonal antibody 1G12, a sensitive radioimmunoassay to detect 1G12 antigen has been developed. The antigen was detected on the surface of peripheral blood mononuclear cells (PBMC). A study was performed to examine the frequency of distribution of this antigen on the surface of PBMC from healthy donors and patients with cancer. An antigen level of above 30 units per 1 x 10(6) PBMC was considered positive. None of 41 healthy donors (0%) and 16 of 96 patients (16.7%) with benign diseases of the lung, ovary, and uterus were positive. In contrast, 27 of 41 patients (65.8%) with lung cancer, 14 of 18 patients (77.8%) with ovarian cancer, and 16 of 27 patients (59.2%) with uterine cervical cancer had elevated levels of 1G12 antigen. When patients were grouped by stages of cancer, PBMC from patients in relatively early stages (stages I and II) also gave positive results, i.e., 9 of 17 patients (59.2%) with lung cancer, 6 of 10 patients (60%) with ovarian cancer, and 11 of 22 patients (50%) with uterine cervical cancer of these stages were positive. These results suggest that the detection of 1G12 antigen on PBMC of patients may be useful for early diagnosis of cancers.