Purified mumps virus (Urabe strain) was given mainly by intravenous injection to a total of 200 patients with cancer. The only adverse clinical reaction was transient mild fever in about half the patients. The beneficial clinical effects were as follows; decrease or disappearance of ascites and edema of the lower limbs at high rates (26/37 and 4/4, respectively), usually within a week after treatment: decrease or stoppage of cancerous bleeding in 30 of 35 patients: decrease or disappearance of pain in most of the patients: and tumor regression in 26 patients with cancer of the breast, rectum, ileocaecum, thyroid gland, uterus, skin, etc. Histologically, the virus-treatment caused shrinkage of nuclei and vacuolization of the cytoplasma of tumor cells, but the degenerative changes of tumor cells were not so great as those after chemotherapy or radiotherapy. Infiltration of lymphocytes, fibrosis and collagenesis occurred around tumor tissues, where necrosis or exfoliation of tumor cells was frequently observed.