Daily intramuscular injections of 3 X 10(6) international units of human leukocyte interferon were given to five patients with advanced ovarian carcinoma, all of whom previously received other forms of treatment. Ascitic fluid production ceases in two out of two patients. According to the criteria specified by Young and DeVita a partial response was observed in one patient and in two other patients the disease was stable for more than 1 year. Side effects of the interferon therapy were relatively mild. The introduction of interferon therapy was followed by an increase in the natural killer cell activity of peripheral blood lymphocytes in all three patients examined. Natural killer cell activity decreased after cessation of interferon therapy in the one patient where this was tested.