IN the course of work on the effects of infection with several viruses upon different kinds of tissue cells from several animal species in vitro, it was found unexpectedly that Rift Valley fever virus has a marked destructive action upon rat sarcoma cells grown in tissue culture. A number of papers have appeared on the effects of infection with viruses upon various tumours in several animal species1. A destructive effect of the viruses of eastern equine encephalomyelitis and poliomyelitis on the tumour cells grown in tissue culture has been demonstrated by Bang and Gey2 and by Scherer, Syverton and Gey3 respectively; but no report of an effect of infection with Rift Valley fever virus upon tumour cells either in vivo or in vitro came to our attention. It is felt, therefore, that the results so far obtained by us are sufficiently interesting to merit publication at this stage.