The impact of basic research on oncology has been particularly impressive in the recent search for the cause of malignancy. Equally impressive is our appreciation of the cause of tumors based on observation. Even in the earliest era of the study of infectious diseases, it was proposed that tumorous growth in animals and birds resulted from "minute" infectious particles. Experiments then supported the hypothesis, that the etiologic agent in many animal tumors was viral. The development of molecular biology, supported by technical advances and conceptual understanding of macromolecular action, led to an explosive increase in studies of animal oncogenic viruses. For a decade, new findings emerged from research laboratories revealing the enormous variety of such agents, the complexity of their interactions with cells, and the tantalizingly possible mechanisms by which they might cause malignant transformation of the cell. Repeatedly, clues emerged which suggested the intervention of viral agents in human tumors. A breathless excitement pervaded both the scientific and public communities as highly publicized findings rapidly followed one another. The excitement was no less scientific than it was practical, for implicit in the concept of the viral oncogen is the possibility of specific virostatic or virotoxic agents or of immunization.