A 60 kd oncofetal protein, shown previously to be present in tumor cytosol and rat embryonic tissue, has been measured by a specific assay in the blood plasma of 42 cancer patients with tumors at 12 different sites. All 42 cancer patient plasmas were positive, while the plasmas from 20 normal controls were negative. Similar results were obtained with the experimental animal system. The protein is present in low concentrations in cancer patient plasma, amounting to less than 0.008% of the total plasma protein. Following complete surgical removal of the tumor, the plasma concentration of the factor drops rapidly. In the rate the biological half-life was 8 days.