In the study of human gastroenteritis, the use of electron microscopy and related techniques has led to the identification of new viral agents which had previously escaped detection by routine cell-culture procedures. Efforts to characterize and further study these agents are currently being made by researchers in many areas of the world. Two of the best known agents, rotavirus and Norwalk virus, have now been implicated in waterborne outbreaks of this illness. Another virus, the Snow Mountain agent, was first identified from the investigation of one waterborne gastroenteritis outbreak.