KB cells from a human nasopharyngeal tumor were cocultivated with human embryonic fibroblasts (HF 8101 cells); 7 to 14 days after incubation, "spongy degeneration"-like changes developed in the target cell-growing area. These changes developed in other target cells [HeLa cells from human cervical cancer, human hepatoma cells (PLC/PRF/5), and human amnion FL cells] cocultured with several kinds of human embryonic fibroblasts [HF 8101 cells, HF 8103 cells, and HEL cells]; however, HF 8101 cells did not cause degenerative changes in murine L929 cells. The degenerative changes were enhanced by treatment with human leukocyte interferon or human fibroblast interferon at a dose of 1,000 or 10,000 IU/ml, but there was no significant difference in the enhancing effect between human leukocyte and human fibroblast interferons. Mouse L929 interferon did not enhance the degenerative changes in KB cells caused by HF 8101 cells. It was concluded that human fibroblasts caused the degenerative changes in the human tumor cells and the continuous cell line and that the changes were enhanced by treatment with either human leukocyte interferon or human fibroblast interferon.