The present invention is concerned with a novel antigens associated with human tumors, including carcinomas of the colon or lung, as well as novel monoclonal antibodies which specifically bind to said antigen. The antibodies bind to normal human cells to a much lesser degree than to tumor cells. The antigens include 100 kDa glycoprotein that has a colorectal cancer membrane bound and a soluble form, has a UV absorbance peak at about 228 nm, an isoelectric point of about 3.5 to 4, a sialic acid content of about 20%, and does not substantially bind to an antibody specific for ACT. The antibodies find use both in methods such as the detection of malignant cells associated with tumors and in monitoring therapeutic treatment of humans with tumors.