Mixed infection, the infection of a single cell by two distinguishable viruses, has been demonstrated by electron microscopy in cultures of African green monkey kidney cells after inoculation with simian virus 40 and herpes simplex. Mixed infection occurs rarely when the two viruses are inoculated simultaneously, but if herpes is inoculated 24 hours after SV40 both viruses are found in the same nucleus in about 5 percent of intact cells.