Gel filtration of urine from Patient ED with acute myelocytic leukemia showed a prominent protein peak with elution position corresponding to molecular weights of 20,000 to 35,000. The protein (EDC1) was isolated in pure form by sequential gel filtration and ion-exchange chromatography. Molecular weight of purified EDC1 was 27,000; it contained 27% carbohydrate and was rich in half-cystine (5% of residues). EDC1 was antigenically and chemically distinct from the recognized glycoproteins of normal plasma. With a specific rabbit antiserum and 125l-labeled EDC1, a radioimmunoassay for the glycoprotein was developed. Both noncancer and cancer plasmas contained immunoreactive material. In noncancer plasma, all the immunoreactivity was eluted from Sephadex G-75 and G-200 in position corresponding to molecular weights of 60,000 to 100,000 (Peak 1). In cancer plasma, an additional peak of immunoreactivity was eluted in the position corresponding to EDC1 (M.W., 20,000 to 30,000; Peak 2). Eighty-six % of urines from patients without clinical cancer were nonreactive in radioimmunoassay (less than 0.1 microgram immunoreactive EDC1 per ml); 11 and 3%, respectively, contained immunoreactivity equivalent to 0.1 to 0.9 and 1 to 9 microgram EDC1 per mi, entirely of Peak 1 type. Ninety-one % of urines from patients with disseminated cancer contained immunoreactivity equivalent to 10 to 9,999 microgram EDC1 per ml, primarily of Peak 2 type.