Pregnancy-associated α₂-glycoprotein (α₂-PAG) levels were measured in several kinds of samples by a modification of Laurell's electroimmunoassay using a rabbit anti-α₂-PAG serum. Samples were sera obtained from healthy females (46 cases), patients with benign ovarian tumors (8 cases), patients with ovarian cancers (18 cases), nude mice bearing heterotransplanted human ovarian cancers (4 cases), and ascites and urine of cancer patients. Serum α₂-PAG levels in patients with benign ovarian tumors were 3.8 ± 3.2 U/μl and almost the same as those of controls (2.6 ± 3.0 U/μl). However, serum α₂-PAG levels were higher in patients with ovarian cancers (12.0 ± 3.7 U/μl). α₂-PAG was scarcely detected in ascites or urine obtained from cancer patients, and was not detected in sera of tumor-bearing nude mice, despite the high levels of α₂-PAG in those of original patients. The reliability of α₂-PAG as a tumor-associated marker in patients with ovarian cancer was reinforced by the comparison of the positive rates of the two parameters, α₂-PAG and carcinoembryonic antigen (CEA).