Effective vaccines are not available for most viral diseases. This situation may soon change when the full force of contemporary molecular biology is applied to immunoprophylaxis. In certain viral diseases, particularly those affecting the respiratory and gastrointestinal tracts, live attenuated vaccines are needed to confer effective protection. Until now the major obstacle to success has been genetic instability. It may be possible to construct stable, satisfactorily attenuated mutants by cloning viral DNA or RNA and then subjecting the cloned DNA to enzyme surgery to create viable deletion mutations. Modified cloned DNA derived from positive-strand viral RNA could then be transferred back into its virus by transfection of cells. Conversion of mutant cloned DNA into negative-strand RNA and transfer into its virus will require a more elaborate type of rescue.