The effect on tumour growth of adding heavily irradiated cells to a standard number of viable mouse melanoma tumour cells before its inoculation into the syngeneic host was investigated. It was found that admixed irradiated syngeneic tumour cells stimulated tumour growth. Irradiated syngeneic fibroblasts had no effect. Irradiated tumour cells with a strong histocompatability difference inhibited tumour growth. It is possible that this inhibitory effect was due to a stimulation of the host immune system.