Severe etch virus prevents die multiplication of potato virus Y andHyoscyamusvirus 3 and replaces them even in plants in which they are established. Mild etch virus reduces the concentration of potato virus Y but does not suppress it completely. Cucumber virus 1 multiplies normally in mixed infections with any of the three other insect‐transmitted viruses. Possible implications of these results on the mechanism of virus multiplication are discussed; it is suggested that these viruses inactivate in cell sap at approximately the same rate as they denature in vitro.No differences were found between the stability of antibodies to viruses with different propertie