The explosion of new information concerning the influenza A viral genome provides a basis for deliberate manipulation of its genes with the intent of introducing specific mutations that render influenza virus attenuated and useful for prevention of disease. Currently there is considerable effort to develop a defined set of mutant genes that confer a specific and desired level of attenuation upon any viral recombinant into which they are transferred. In this manner new antigenic variants of influenza A virus may be satisfactorily attenuated after transfer of the mutant genes. The mutant genes must be readily identifiable by simple in-vitro techniques, thus enabling the genetic basis of attenuation to be monitored directly during all phases of vaccine development, manufacture and utilization in man. We describe our experience with two sets of ts mutant genes which affect viral RNA transcription or synthesis and which effect a reproducible level of attenuation in wild-type influenza A virus.