90K/Mac-2 BP (90 kDa) is a human serum glycoprotein, originally identified in the supernatant of human breast cancer cells.Serum levels of 90K/Mac-2 BP have been determined in patients with various forms of neoplasia and with some viral infections. In breast and colorectal cancer and in non-Hodgkin lymphoma, high levels of 90K/Mac-2 BP are correlated with a poor prognosis. The determination of 90K/Mac-2 BP in the serum can be used as a valuable parameter for monitoring the outcome of cancer and viral infections.