In rats given implants of Walker tumor a sizeable increase in tumor mass has been shown to precede significant increases in the seromucoid values. Early irradiation to the tumor has prolonged the survival days, and then seromucoid values have remained within the normal limits. After the occurrence of metastatic growth, irradiation to the primary tumor has repressed the tumor growth but the seromucoid has increased significantly. If distant metastasis has occurred, the seromucoid values increase significantly. Therefore the usefulness of the seromucoid value in assessing the occurrence of metastatic growth is shown. The seromucoid values in rats given 300 rad partial body (the right hind leg) irradiation daily for 20 days have been within the normal limits. However, the seromucoid values in rats given 1,000 rad whole body irradiation at a time have gradually increased up to the 3rd day, and markedly increased in the 5th day after irradiation. The seromucoid values on the 5th day after irradiation were about 6.4 times as much as those of normal rats.