Total protein and sialic acid levels were determined in the supernatant of serum treated with perchloric acid. Patients with either localized or advanced metastatic malignancy have significantly elevated mean serum values. The highest levels occur in patients with lung, GI, GYN cancer, lymphoma and malignant melanoma. Patients with leukemia and multiple myeloma have slightly elevated values, but they were not significantly different from normal. Patients following curative surgery have normal values while patients in clinical remission following chemotherapy have elevated mean serum protein and NANA levels. Elevated values also occur in patients with benign tumors and 12% of patients with nonmalignant disease. Tumor cells appear to shed macromolecules which contribute to the observed elevation of serum protein and sialic acid levels.