Renal cell carcinoma cells produced the substance(s) which killed them (suicide factor(s)) after co-culture with mumps virus. The suicide factor(s) were heat-sensitive and were degraded with trypsin. Furthermore, actinomycin D inhibited the production of the substance(s) by cancer cells. Considering these facts, the substance(s) were thought to be protein(s) derived from de novo synthesis in cancer cells. It was demonstrated that renal cell carcinoma cells proliferated with the autocrine loop of interleukin-6 (IL-6). Mumps virus almost completely inhibited the IL-6 production in several hours. Because of these two facts, the suicide process might be initiated in renal cell carcinoma cells after encountering mumps virus, i.e. inhibition of the autocrine growth loop of IL-6 followed by the induction of an autocrine killing loop of unknown substance(s).